Move to start of current header in diff-{file,hunk}-prev

If point was after a file or hunk header, the diff-file-prev and
diff-hunk-prev commands would move to the start of that header.
But if point was *within* the header, they would not move, and
would report "No previous file" or "No previous hunk".  This
differs from the behavior of most other movement commands,
e.g. backward-sexp or backward-sentence.

This commit fixes diff-file-prev and diff-hunk-prev, as well as
other easy-mmode-define-navigation BASE-prev commands.  Now
these commands move to the start of the containing "thing" just
like other movement commands.

* lisp/emacs-lisp/easy-mmode.el (easy-mmode--prev):
Move to start of current match first. (bug#73172)
* etc/NEWS: Document the behavior change.

2 files changed, 17 insertions, 0 deletions

diff --git a/etc/NEWS b/etc/NEWS
index 85504afa1a4..76c58c0f269 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -370,6 +370,12 @@ hunk), and then removes the hunk from the diffs.
 This is useful to undo or revert changes, committed and uncommitted, when
 you are in buffers generated by 'C-x v =' and 'C-x v D'.
 
+---
+*** diff-file-prev and diff-hunk-prev reliably move to start of header.
+Previously, diff-file-prev and diff-hunk-prev would move when point is
+after the corresponding file or hunk header, but not when inside it.
+Now they will reliably move to the start of the current header.
+
 ** php-ts-mode
 
 ---
diff --git a/lisp/emacs-lisp/easy-mmode.el b/lisp/emacs-lisp/easy-mmode.el
index d3dcab899d6..7a94d832273 100644
--- a/lisp/emacs-lisp/easy-mmode.el
+++ b/lisp/emacs-lisp/easy-mmode.el
@@ -772,6 +772,17 @@ ENDFUN and NARROWFUN are treated like in `easy-mmode-define-navigation'."
   (unless count (setq count 1))
   (if (< count 0) (easy-mmode--next re name (- count) endfun narrowfun)
     (let ((re-narrow (and narrowfun (prog1 (buffer-narrowed-p) (widen)))))
+      ;; If point is inside a match for RE, move to its beginning like
+      ;; `backward-sexp' and other movement commands.
+      (when (and (not (zerop count))
+                 (save-excursion
+                   ;; Make sure we're out of the current match if any.
+                   (goto-char (if (re-search-backward re nil t 1)
+                                  (match-end 0) (point-min)))
+                   (re-search-forward re nil t 1))
+                 (< (match-beginning 0) (point) (match-end 0)))
+        (goto-char (match-beginning 0))
+        (setq count (1- count)))
       (unless (re-search-backward re nil t count)
         (user-error "No previous %s" name))
       (when re-narrow (funcall narrowfun)))))