diff kitten: Add support for files that are identical apart from mode changes

Fixes #6611

docs/changelog.rst

@@ -78,6 +78,8 @@ Detailed list of changes
 
 - ssh kitten: Fix a regression causing hostname directives in :file:`ssh.conf` not matching when username is specified (:disc:`6609`)
 
+- diff kitten: Add support for files that are identical apart from mode changes (:iss:`6611`)
+
 
 0.29.2 [2023-07-27]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

kittens/diff/collect.go

@@ -312,6 +312,16 @@ func (self *Collection) collect_files(left, right string) error {
 		if ld != rd {
 			changed_names.Add(n)
 			self.add_change(left_path_map[n], right_path_map[n])
+		} else {
+			if lstat, err := os.Stat(left_path_map[n]); err == nil {
+				if rstat, err := os.Stat(right_path_map[n]); err == nil {
+					if lstat.Mode() != rstat.Mode() {
+						// identical files with only a mode change
+						changed_names.Add(n)
+						self.add_change(left_path_map[n], right_path_map[n])
+					}
+				}
+			}
 		}
 	}
 	removed := left_names.Subtract(common_names)

kittens/diff/render.go

@@ -6,6 +6,7 @@ import (
 	"errors"
 	"fmt"
 	"math"
+	"os"
 	"strconv"
 	"strings"
 
@@ -571,7 +572,15 @@ func lines_for_diff(left_path string, right_path string, patch *Patch, columns,
 		is_full_width: true,
 	}
 	if patch.Len() == 0 {
-		for _, line := range splitlines("The files are identical", columns-margin_size) {
+		txt := "The files are identical"
+		if lstat, err := os.Stat(left_path); err == nil {
+			if rstat, err := os.Stat(right_path); err == nil {
+				if lstat.Mode() != rstat.Mode() {
+					txt = fmt.Sprintf("Mode changed: %s to %s", lstat.Mode(), rstat.Mode())
+				}
+			}
+		}
+		for _, line := range splitlines(txt, columns-margin_size) {
 			sl := ScreenLine{}
 			sl.left.marked_up_text = line
 			ht.screen_lines = append(ht.screen_lines, &sl)