fix/reverse: keep "Only in" entries when reversing

diff/reverse.go

@@ -26,6 +26,10 @@ func ReverseFileDiff(fd *FileDiff) (*FileDiff, error) {
 		NewTime:  fd.OrigTime,
 		Extended: extended,
 	}
+	// "Only in" lines read the same in both directions.
+	if fd.NewName == "" {
+		reverse.OrigName, reverse.NewName = fd.OrigName, fd.NewName
+	}
 	for _, hunk := range fd.Hunks {
 		invHunk, err := reverseHunk(hunk)
 		if err != nil {

diff/reverse_test.go

@@ -116,6 +116,10 @@ func TestReverseMultiFileDiff(t *testing.T) {
 			inputFile: "sample_multi_file.diff",
 			wantFile:  "sample_multi_file.reversed",
 		},
+		{
+			inputFile: "sample_contains_added_deleted_files.diff",
+			wantFile:  "sample_contains_added_deleted_files.reversed",
+		},
 	}
 	for _, test := range tests {
 		inputData, err := os.ReadFile(filepath.Join("testdata", test.inputFile))

diff/testdata/sample_contains_added_deleted_files.reversed

@@ -0,0 +1,11 @@
+diff -u source_a/file_1.txt  source_b/file_1.txt
+--- source_b/file_1.txt
++++ source_a/file_1.txt
+@@ -3,4 +2,3 @@
+ To be, or not to be, that is the question:
+-The slings and arrows of outrageous fortune,
+-Or to take arms against a sea of troubles
++Whether 'tis nobler in the mind to suffer
+ And by opposing end them. To die—to sleep,
+Only in source_a: file_2.txt
+Only in source_b: file_3.txt