Add a user config for using git's external diff command for paging (#4832)

### PR Description

This is similar to using lazygit's `Git.Paging.ExternalDiffCommand`
config, except that the command is configured in git. This can be done
either with git's `diff.external` config, or through `.gitattributes`,
so it gives a bit more flexibility.

We could consider removing the `Git.Paging.ExternalDiffCommand` config
now, because its functionality is covered by the new config. I decided
to keep it though, because I don't want to make this a breaking change,
and also because some users might want to have the external diff command
only in lazygit but not on the command line.

docs/Config.md

@@ -306,6 +306,9 @@ git:
     # e.g. 'difft --color=always'
     externalDiffCommand: ""
 
+    # If true, Lazygit will use git's `diff.external` config for paging. The advantage over `externalDiffCommand` is that this can be configured per file type in .gitattributes; see https://git-scm.com/docs/gitattributes#_defining_an_external_diff_driver.
+    useExternalDiffGitConfig: false
+
   # Config relating to committing
   commit:
     # If true, pass '--signoff' flag when committing

docs/Custom_Pagers.md

@@ -74,3 +74,13 @@ git:
   paging:
     externalDiffCommand: difft --color=always --display=inline --syntax-highlight=off
 ```
+
+Instead of setting this command in lazygit's `externalDiffCommand` config, you can also tell lazygit to use the external diff command that is configured in git itself (`diff.external`), by using
+
+```yaml
+git:
+  paging:
+    useExternalDiffGitConfig: true
+```
+
+This can be useful if you also want to use it for diffs on the command line, and it also has the advantage that you can configure it per file type in `.gitattributes`; see https://git-scm.com/docs/gitattributes#_defining_an_external_diff_driver.

pkg/commands/git_commands/commit.go

@@ -257,10 +257,11 @@ func (self *CommitCommands) ShowCmdObj(hash string, filterPaths []string) *oscom
 	contextSize := self.UserConfig().Git.DiffContextSize
 
 	extDiffCmd := self.UserConfig().Git.Paging.ExternalDiffCommand
+	useExtDiffGitConfig := self.UserConfig().Git.Paging.UseExternalDiffGitConfig
 	cmdArgs := NewGitCmd("show").
 		Config("diff.noprefix=false").
 		ConfigIf(extDiffCmd != "", "diff.external="+extDiffCmd).
-		ArgIfElse(extDiffCmd != "", "--ext-diff", "--no-ext-diff").
+		ArgIfElse(extDiffCmd != "" || useExtDiffGitConfig, "--ext-diff", "--no-ext-diff").
 		Arg("--submodule").
 		Arg("--color="+self.UserConfig().Git.Paging.ColorArg).
 		Arg(fmt.Sprintf("--unified=%d", contextSize)).

pkg/commands/git_commands/commit_test.go

@@ -256,6 +256,7 @@ func TestCommitShowCmdObj(t *testing.T) {
 		similarityThreshold int
 		ignoreWhitespace    bool
 		extDiffCmd          string
+		useExtDiffGitConfig bool
 		expected            []string
 	}
 
@@ -314,6 +315,15 @@ func TestCommitShowCmdObj(t *testing.T) {
 			extDiffCmd:          "difft --color=always",
 			expected:            []string{"-C", "/path/to/worktree", "-c", "diff.external=difft --color=always", "-c", "diff.noprefix=false", "show", "--ext-diff", "--submodule", "--color=always", "--unified=3", "--stat", "--decorate", "-p", "1234567890", "--find-renames=50%", "--"},
 		},
+		{
+			testName:            "Show diff using git's external diff config",
+			filterPaths:         []string{},
+			contextSize:         3,
+			similarityThreshold: 50,
+			ignoreWhitespace:    false,
+			useExtDiffGitConfig: true,
+			expected:            []string{"-C", "/path/to/worktree", "-c", "diff.noprefix=false", "show", "--ext-diff", "--submodule", "--color=always", "--unified=3", "--stat", "--decorate", "-p", "1234567890", "--find-renames=50%", "--"},
+		},
 	}
 
 	for _, s := range scenarios {
@@ -323,6 +333,7 @@ func TestCommitShowCmdObj(t *testing.T) {
 			userConfig.Git.IgnoreWhitespaceInDiffView = s.ignoreWhitespace
 			userConfig.Git.DiffContextSize = s.contextSize
 			userConfig.Git.RenameSimilarityThreshold = s.similarityThreshold
+			userConfig.Git.Paging.UseExternalDiffGitConfig = s.useExtDiffGitConfig
 
 			runner := oscommands.NewFakeRunner(t).ExpectGitArgs(s.expected, "", nil)
 			repoPaths := RepoPaths{

pkg/commands/git_commands/diff.go

@@ -21,13 +21,14 @@ func NewDiffCommands(gitCommon *GitCommon) *DiffCommands {
 func (self *DiffCommands) DiffCmdObj(diffArgs []string) *oscommands.CmdObj {
 	extDiffCmd := self.UserConfig().Git.Paging.ExternalDiffCommand
 	useExtDiff := extDiffCmd != ""
+	useExtDiffGitConfig := self.UserConfig().Git.Paging.UseExternalDiffGitConfig
 	ignoreWhitespace := self.UserConfig().Git.IgnoreWhitespaceInDiffView
 
 	return self.cmd.New(
 		NewGitCmd("diff").
 			Config("diff.noprefix=false").
 			ConfigIf(useExtDiff, "diff.external="+extDiffCmd).
-			ArgIfElse(useExtDiff, "--ext-diff", "--no-ext-diff").
+			ArgIfElse(useExtDiff || useExtDiffGitConfig, "--ext-diff", "--no-ext-diff").
 			Arg("--submodule").
 			Arg(fmt.Sprintf("--color=%s", self.UserConfig().Git.Paging.ColorArg)).
 			ArgIf(ignoreWhitespace, "--ignore-all-space").

pkg/commands/git_commands/stash.go

@@ -82,6 +82,7 @@ func (self *StashCommands) Hash(index int) (string, error) {
 
 func (self *StashCommands) ShowStashEntryCmdObj(index int) *oscommands.CmdObj {
 	extDiffCmd := self.UserConfig().Git.Paging.ExternalDiffCommand
+	useExtDiffGitConfig := self.UserConfig().Git.Paging.UseExternalDiffGitConfig
 
 	// "-u" is the same as "--include-untracked", but the latter fails in older git versions for some reason
 	cmdArgs := NewGitCmd("stash").Arg("show").
@@ -89,7 +90,7 @@ func (self *StashCommands) ShowStashEntryCmdObj(index int) *oscommands.CmdObj {
 		Arg("--stat").
 		Arg("-u").
 		ConfigIf(extDiffCmd != "", "diff.external="+extDiffCmd).
-		ArgIfElse(extDiffCmd != "", "--ext-diff", "--no-ext-diff").
+		ArgIfElse(extDiffCmd != "" || useExtDiffGitConfig, "--ext-diff", "--no-ext-diff").
 		Arg(fmt.Sprintf("--color=%s", self.UserConfig().Git.Paging.ColorArg)).
 		Arg(fmt.Sprintf("--unified=%d", self.UserConfig().Git.DiffContextSize)).
 		ArgIf(self.UserConfig().Git.IgnoreWhitespaceInDiffView, "--ignore-all-space").

pkg/commands/git_commands/stash_test.go

@@ -104,6 +104,7 @@ func TestStashStashEntryCmdObj(t *testing.T) {
 		similarityThreshold int
 		ignoreWhitespace    bool
 		extDiffCmd          string
+		useExtDiffGitConfig bool
 		expected            []string
 	}
 
@@ -141,6 +142,15 @@ func TestStashStashEntryCmdObj(t *testing.T) {
 			extDiffCmd:          "difft --color=always",
 			expected:            []string{"git", "-C", "/path/to/worktree", "-c", "diff.external=difft --color=always", "stash", "show", "-p", "--stat", "-u", "--ext-diff", "--color=always", "--unified=3", "--find-renames=50%", "refs/stash@{5}"},
 		},
+		{
+			testName:            "Show diff using git's external diff config",
+			index:               5,
+			contextSize:         3,
+			similarityThreshold: 50,
+			ignoreWhitespace:    false,
+			useExtDiffGitConfig: true,
+			expected:            []string{"git", "-C", "/path/to/worktree", "stash", "show", "-p", "--stat", "-u", "--ext-diff", "--color=always", "--unified=3", "--find-renames=50%", "refs/stash@{5}"},
+		},
 		{
 			testName:            "Default case",
 			index:               5,
@@ -158,6 +168,7 @@ func TestStashStashEntryCmdObj(t *testing.T) {
 			userConfig.Git.DiffContextSize = s.contextSize
 			userConfig.Git.RenameSimilarityThreshold = s.similarityThreshold
 			userConfig.Git.Paging.ExternalDiffCommand = s.extDiffCmd
+			userConfig.Git.Paging.UseExternalDiffGitConfig = s.useExtDiffGitConfig
 			repoPaths := RepoPaths{
 				worktreePath: "/path/to/worktree",
 			}

pkg/commands/git_commands/working_tree.go

@@ -267,10 +267,11 @@ func (self *WorkingTreeCommands) WorktreeFileDiffCmdObj(node models.IFile, plain
 	noIndex := !node.GetIsTracked() && !node.GetHasStagedChanges() && !cached && node.GetIsFile()
 	extDiffCmd := self.UserConfig().Git.Paging.ExternalDiffCommand
 	useExtDiff := extDiffCmd != "" && !plain
+	useExtDiffGitConfig := self.UserConfig().Git.Paging.UseExternalDiffGitConfig
 
 	cmdArgs := NewGitCmd("diff").
 		ConfigIf(useExtDiff, "diff.external="+extDiffCmd).
-		ArgIfElse(useExtDiff, "--ext-diff", "--no-ext-diff").
+		ArgIfElse(useExtDiff || useExtDiffGitConfig, "--ext-diff", "--no-ext-diff").
 		Arg("--submodule").
 		Arg(fmt.Sprintf("--unified=%d", contextSize)).
 		Arg(fmt.Sprintf("--color=%s", colorArg)).
@@ -304,11 +305,12 @@ func (self *WorkingTreeCommands) ShowFileDiffCmdObj(from string, to string, reve
 
 	extDiffCmd := self.UserConfig().Git.Paging.ExternalDiffCommand
 	useExtDiff := extDiffCmd != "" && !plain
+	useExtDiffGitConfig := self.UserConfig().Git.Paging.UseExternalDiffGitConfig
 
 	cmdArgs := NewGitCmd("diff").
 		Config("diff.noprefix=false").
 		ConfigIf(useExtDiff, "diff.external="+extDiffCmd).
-		ArgIfElse(useExtDiff, "--ext-diff", "--no-ext-diff").
+		ArgIfElse(useExtDiff || useExtDiffGitConfig, "--ext-diff", "--no-ext-diff").
 		Arg("--submodule").
 		Arg(fmt.Sprintf("--unified=%d", contextSize)).
 		Arg("--no-renames").

pkg/config/user_config.go

@@ -318,6 +318,8 @@ type PagingConfig struct {
 	Pager PagerType `yaml:"pager"`
 	// e.g. 'difft --color=always'
 	ExternalDiffCommand string `yaml:"externalDiffCommand"`
+	// If true, Lazygit will use git's `diff.external` config for paging. The advantage over `externalDiffCommand` is that this can be configured per file type in .gitattributes; see https://git-scm.com/docs/gitattributes#_defining_an_external_diff_driver.
+	UseExternalDiffGitConfig bool `yaml:"useExternalDiffGitConfig"`
 }
 
 type CommitConfig struct {

schema/config.json

@@ -1675,6 +1675,11 @@
         "externalDiffCommand": {
           "type": "string",
           "description": "e.g. 'difft --color=always'"
+        },
+        "useExternalDiffGitConfig": {
+          "type": "boolean",
+          "description": "If true, Lazygit will use git's `diff.external` config for paging. The advantage over `externalDiffCommand` is that this can be configured per file type in .gitattributes; see https://git-scm.com/docs/gitattributes#_defining_an_external_diff_driver.",
+          "default": false
         }
       },
       "additionalProperties": false,