Fix panic on short "diff --git" args

diff/parse.go

@@ -466,7 +466,7 @@ func parseDiffGitArgs(diffArgs string) (string, string, bool) {
 				return first, second, true
 			}
 			// If the names don't have the a/ and b/ prefixes and they're equal, proceed.
-			if !(first[:2] == "a/" && second[:2] == "b/") && first == second {
+			if !(strings.HasPrefix(first, "a/") && strings.HasPrefix(second, "b/")) && first == second {
 				return first, second, true
 			}
 		}

diff/parse_test.go

@@ -91,3 +91,27 @@ func TestParseDiffGitArgs_Unsuccessful(t *testing.T) {
 		}
 	}
 }
+
+// Short, space-heavy args reach the ambiguous unquoted-with-spaces branch with a
+// filename shorter than the "a/"/"b/" prefixes it is compared against. The syntax
+// is valid but no filename can be extracted, so the function must return
+// ("", "", true) rather than panicking on the prefix slice.
+func TestParseDiffGitArgs_ShortAmbiguous(t *testing.T) {
+	for _, input := range []string{`x  `, `a  `, `ab   `} {
+		first, second, success := parseDiffGitArgs(input)
+		if !success || first != "" || second != "" {
+			t.Errorf("`diff --git %s`: expected (\"\", \"\", true), got (%q, %q, %v)", input, first, second, success)
+		}
+	}
+}
+
+func TestParseFileDiff_ShortAmbiguousDiffGitArgs(t *testing.T) {
+	input := []byte("diff --git x  \ndeleted file mode 100644\nindex e69de29..0000000\n")
+	fileDiff, err := ParseFileDiff(input)
+	if err != nil {
+		t.Fatalf("ParseFileDiff: expected success, got %s", err)
+	}
+	if fileDiff.OrigName != "" || fileDiff.NewName != "/dev/null" {
+		t.Errorf("ParseFileDiff: expected empty original name and /dev/null new name, got %q and %q", fileDiff.OrigName, fileDiff.NewName)
+	}
+}