Don't use hunk mode for added or deleted files even when useHunkModeInStagingView config is on (#4758)

- **PR Description**

When entering staging (or patch building) for an added or deleted file,
it doesn't make sense to use hunk mode, because pressing space would
stage/unstage the entire file, and if the user wanted to do that, they
would have pressed space in the Files panel. So always use line mode for
added/deleted files by default, even if the useHunkModeInStagingView
user config is on.

pkg/commands/patch/patch.go

@@ -186,3 +186,17 @@ func (self *Patch) AdjustLineNumber(lineNumber int) int {
 
 	return adjustedLineNumber
 }
+
+func (self *Patch) IsSingleHunkForWholeFile() bool {
+	if len(self.hunks) != 1 {
+		return false
+	}
+
+	// We consider a patch to be a single hunk for the whole file if it has only additions or
+	// deletions but not both, and no context lines. This not quite correct, because it will also
+	// return true for a block of added or deleted lines if the diff context size is 0, but in this
+	// case you wouldn't be able to stage things anyway, so it doesn't matter.
+	bodyLines := self.hunks[0].bodyLines
+	return nLinesWithKind(bodyLines, []PatchLineKind{DELETION, CONTEXT}) == 0 ||
+		nLinesWithKind(bodyLines, []PatchLineKind{ADDITION, CONTEXT}) == 0
+}

pkg/commands/patch/patch_test.go

@@ -115,6 +115,17 @@ index 0000000..4e680cc
 +grape
 `
 
+const deletedFile = `diff --git a/newfile b/newfile
+deleted file mode 100644
+index 4e680cc1f..000000000
+--- a/newfile
++++ /dev/null
+@@ -1,3 +0,0 @@
+-apple
+-orange
+-grape
+`
+
 const addNewlineToPreviouslyEmptyFile = `diff --git a/newfile b/newfile
 index e69de29..c6568ea 100644
 --- a/newfile
@@ -554,6 +565,10 @@ func TestParseAndFormatPlain(t *testing.T) {
 			testName: "newFile",
 			patchStr: newFile,
 		},
+		{
+			testName: "deletedFile",
+			patchStr: deletedFile,
+		},
 		{
 			testName: "addNewlineToPreviouslyEmptyFile",
 			patchStr: addNewlineToPreviouslyEmptyFile,
@@ -695,3 +710,64 @@ func TestAdjustLineNumber(t *testing.T) {
 		})
 	}
 }
+
+func TestIsSingleHunkForWholeFile(t *testing.T) {
+	scenarios := []struct {
+		testName       string
+		patchStr       string
+		expectedResult bool
+	}{
+		{
+			testName:       "simpleDiff",
+			patchStr:       simpleDiff,
+			expectedResult: false,
+		},
+		{
+			testName:       "addNewlineToEndOfFile",
+			patchStr:       addNewlineToEndOfFile,
+			expectedResult: false,
+		},
+		{
+			testName:       "removeNewlinefromEndOfFile",
+			patchStr:       removeNewlinefromEndOfFile,
+			expectedResult: false,
+		},
+		{
+			testName:       "twoHunks",
+			patchStr:       twoHunks,
+			expectedResult: false,
+		},
+		{
+			testName:       "twoChangesInOneHunk",
+			patchStr:       twoChangesInOneHunk,
+			expectedResult: false,
+		},
+		{
+			testName:       "newFile",
+			patchStr:       newFile,
+			expectedResult: true,
+		},
+		{
+			testName:       "deletedFile",
+			patchStr:       deletedFile,
+			expectedResult: true,
+		},
+		{
+			testName:       "addNewlineToPreviouslyEmptyFile",
+			patchStr:       addNewlineToPreviouslyEmptyFile,
+			expectedResult: true,
+		},
+		{
+			testName:       "exampleHunk",
+			patchStr:       exampleHunk,
+			expectedResult: false,
+		},
+	}
+
+	for _, s := range scenarios {
+		t.Run(s.testName, func(t *testing.T) {
+			patch := Parse(s.patchStr)
+			assert.Equal(t, s.expectedResult, patch.IsSingleHunkForWholeFile())
+		})
+	}
+}

pkg/gui/patch_exploring/state.go

@@ -67,7 +67,7 @@ func NewState(diff string, selectedLineIdx int, view *gocui.View, oldState *Stat
 	}
 
 	selectMode := LINE
-	if useHunkModeByDefault {
+	if useHunkModeByDefault && !patch.IsSingleHunkForWholeFile() {
 		selectMode = HUNK
 	}