fix: match diff delete for empty file

GNU diff -u prints +0,0 for the special case of deleting into an empty file.

Co-authored-by: aymanbagabas <[email protected]>

.github/UPSTREAM

@@ -1 +1 @@
-e5c9e6317588e6bedd8e80531f40267b8248f3c8
+5283a0178901bd81703e379b9f8339457c277205

difftest/difftest.go

@@ -119,6 +119,17 @@ var TestCases = []struct {
 \ No newline at end of file
 `[1:],
 	Edits: []diff.Edit{{Start: 0, End: 1, New: "B"}},
+}, {
+	Name: "delete_empty",
+	In:   "meow",
+	Out:  "", // GNU diff -u special case: +0,0
+	Unified: UnifiedPrefix + `
+@@ -1 +0,0 @@
+-meow
+\ No newline at end of file
+`[1:],
+	Edits:     []diff.Edit{{Start: 0, End: 4, New: ""}},
+	LineEdits: []diff.Edit{{Start: 0, End: 4, New: ""}},
 }, {
 	Name: "append_empty",
 	In:   "", // GNU diff -u special case: -0,0

lcs/doc.go

@@ -151,6 +151,6 @@ be recomputed.
 and can be found at
 http://www.xmailserver.org/diff2.pdf
 
-(There is a generic implementation of the algorithm the the repository with git hash
+(There is a generic implementation of the algorithm the repository with git hash
 b9ad7e4ade3a686d608e44475390ad428e60e7fc)
 */

unified.go

@@ -226,6 +226,9 @@ func (u unified) String() string {
 		}
 		if toCount > 1 {
 			fmt.Fprintf(b, " +%d,%d", hunk.ToLine, toCount)
+		} else if hunk.ToLine == 1 && toCount == 0 {
+			// Match odd GNU diff -u behavior adding to empty file.
+			fmt.Fprintf(b, " +0,0")
 		} else {
 			fmt.Fprintf(b, " +%d", hunk.ToLine)
 		}