Refactor repo_paths.go to use git rev-parse

This changes GetRepoPaths() to pull information from `git rev-parse`
instead of effectively reimplementing git's logic for pathfinding. This
change fixes issues with bare repos, esp. versioned homedir use cases,
by aligning lazygit's path handling to what git itself does.

This change also enables lazygit to run from arbitrary subdirectories of
a repository, including correct handling of symlinks, including "deep"
symlinks into a repo, worktree, a repo's submodules, etc.

Integration tests are now resilient against unintended side effects from
the host's environment variables. Of necessity, $PATH and $TERM are the
only env vars allowed through now.

pkg/integration/tests/worktree/symlink_into_repo_subdir.go

@@ -0,0 +1,63 @@
+package worktree
+
+import (
+	"github.com/jesseduffield/lazygit/pkg/config"
+	. "github.com/jesseduffield/lazygit/pkg/integration/components"
+)
+
+var SymlinkIntoRepoSubdir = NewIntegrationTest(NewIntegrationTestArgs{
+	Description:  "Open lazygit in a symlink into a repo's subdirectory",
+	ExtraCmdArgs: []string{},
+	Skip:         false,
+	SetupConfig: func(config *config.AppConfig) {
+		config.UserConfig.Gui.ShowFileTree = false
+	},
+	SetupRepo: func(shell *Shell) {
+		// we're going to have a directory structure like this:
+		// project
+		//  - repo/a/b/c (main worktree with subdirs)
+		//  - link (symlink to repo/a/b/c)
+		//
+		shell.CreateFileAndAdd("a/b/c/blah", "blah\n")
+		shell.Commit("initial commit")
+		shell.UpdateFile("a/b/c/blah", "updated content\n")
+
+		shell.Chdir("..")
+		shell.RunCommand([]string{"ln", "-s", "repo/a/b/c", "link"})
+
+		shell.Chdir("link")
+	},
+	Run: func(t *TestDriver, keys config.KeybindingConfig) {
+		t.Views().Branches().
+			Lines(
+				Contains("master"),
+			)
+
+		t.Views().Commits().
+			Lines(
+				Contains("initial commit"),
+			)
+
+		t.Views().Files().
+			IsFocused().
+			Lines(
+				Contains(" M a/b/c/blah"), // shows as modified
+			).
+			PressPrimaryAction().
+			Press(keys.Files.CommitChanges)
+
+		t.ExpectPopup().CommitMessagePanel().
+			Title(Equals("Commit summary")).
+			Type("Add blah").
+			Confirm()
+
+		t.Views().Files().
+			IsEmpty()
+
+		t.Views().Commits().
+			Lines(
+				Contains("Add blah"),
+				Contains("initial commit"),
+			)
+	},
+})