Update diff-hl--update to synchronize overlays across indirect buffers

Identify the base buffer of the current file and iterate through all live
buffers to apply diff highlighting to any indirect clones. Since overlays are
buffer-local, this ensures that changes are visually reflected across all views
of the same file while maintaining accurate point-dependent calculations in
each.

Author: jamescherti

diff-hl.el

@@ -740,17 +740,23 @@ Return a list of line overlays used."
         reference
         (lambda (ref-changes)
           (let ((ref-changes (diff-hl-adjust-changes ref-changes changes))
-                reuse)
-            (with-current-buffer orig
-              (diff-hl-remove-overlays)
-              (let ((diff-hl-highlight-function
-                     diff-hl-highlight-reference-function)
-                    (diff-hl-fringe-face-function
-                     diff-hl-fringe-reference-face-function))
-                (setq reuse (diff-hl--update-overlays ref-changes nil)))
-              (diff-hl--update-overlays changes reuse)
-              (when (not (or changes ref-changes))
-                (diff-hl--autohide-margin))))))))))
+                (base (or (buffer-base-buffer orig) orig)))
+            (dolist (buf (buffer-list))
+              (when (and (buffer-live-p buf)
+                         (or (eq buf base)
+                             (eq (buffer-base-buffer buf) base))
+                         (buffer-local-value 'diff-hl-mode buf))
+                (with-current-buffer buf
+                  (let (reuse)
+                    (diff-hl-remove-overlays)
+                    (let ((diff-hl-highlight-function
+                           diff-hl-highlight-reference-function)
+                          (diff-hl-fringe-face-function
+                           diff-hl-fringe-reference-face-function))
+                      (setq reuse (diff-hl--update-overlays ref-changes nil)))
+                    (diff-hl--update-overlays changes reuse)
+                    (when (not (or changes ref-changes))
+                      (diff-hl--autohide-margin)))))))))))))
 
 (defun diff-hl--resolve (value-or-buffer cb)
   (if (listp value-or-buffer)