Support amending the last commit using VC-Hg

* lisp/vc/log-edit.el (log-edit--toggle-amend): Extract from
vc-git-log-edit-toggle-amend (bug#34944).

* lisp/vc/vc-hg.el (vc-hg-log-edit-toggle-amend): New function.
Use the aforementioned.
(vc-hg-log-edit-mode-map): New variable.
(vc-hg-log-edit-mode): New major mode.

3 files changed, 55 insertions, 23 deletions

diff --git a/lisp/vc/log-edit.el b/lisp/vc/log-edit.el
index ba5a1a3d572..91e18c1ec5c 100644
--- a/lisp/vc/log-edit.el
+++ b/lisp/vc/log-edit.el
@@ -1087,6 +1087,22 @@ line of MSG."
       (if summary (insert summary "\n\n"))
       (cons (buffer-string) res))))
 
+(defun log-edit--toggle-amend (last-msg-fn)
+  (when (log-edit-toggle-header "Amend" "yes")
+    (goto-char (point-max))
+    (unless (bolp) (insert "\n"))
+    (insert (funcall last-msg-fn))
+    (save-excursion
+      (rfc822-goto-eoh)
+      (forward-line 1)
+      (let ((pt (point)))
+        (and (zerop (forward-line 1))
+             (looking-at "\n\\|\\'")
+             (let ((summary (buffer-substring-no-properties pt (1- (point)))))
+               (skip-chars-forward " \n")
+               (delete-region pt (point))
+               (log-edit-set-header "Summary" summary)))))))
+
 (provide 'log-edit)
 
 ;;; log-edit.el ends here
diff --git a/lisp/vc/vc-git.el b/lisp/vc/vc-git.el
index a921ff1bb88..192e6cf68f6 100644
--- a/lisp/vc/vc-git.el
+++ b/lisp/vc/vc-git.el
@@ -750,7 +750,7 @@ The car of the list is the current branch."
 (declare-function log-edit-mode "log-edit" ())
 (declare-function log-edit-toggle-header "log-edit" (header value))
 (declare-function log-edit-extract-headers "log-edit" (headers string))
-(declare-function log-edit-set-header "log-edit" (header value &optional toggle))
+(declare-function log-edit--toggle-amend "log-edit" (last-msg-fn))
 
 (defun vc-git-log-edit-toggle-signoff ()
   "Toggle whether to add the \"Signed-off-by\" line at the end of
@@ -767,23 +767,12 @@ the commit message."
   "Toggle whether this will amend the previous commit.
 If toggling on, also insert its message into the buffer."
   (interactive)
-  (when (log-edit-toggle-header "Amend" "yes")
-    (goto-char (point-max))
-    (unless (bolp) (insert "\n"))
-    (insert (with-output-to-string
-              (vc-git-command
-               standard-output 1 nil
-               "log" "--max-count=1" "--pretty=format:%B" "HEAD")))
-    (save-excursion
-      (rfc822-goto-eoh)
-      (forward-line 1)
-      (let ((pt (point)))
-        (and (zerop (forward-line 1))
-             (looking-at "\n\\|\\'")
-             (let ((summary (buffer-substring-no-properties pt (1- (point)))))
-               (skip-chars-forward " \n")
-               (delete-region pt (point))
-               (log-edit-set-header "Summary" summary)))))))
+  (log-edit--toggle-amend
+   (lambda ()
+     (with-output-to-string
+       (vc-git-command
+        standard-output 1 nil
+        "log" "--max-count=1" "--pretty=format:%B" "HEAD")))))
 
 (defvar vc-git-log-edit-mode-map
   (let ((map (make-sparse-keymap "Git-Log-Edit")))
diff --git a/lisp/vc/vc-hg.el b/lisp/vc/vc-hg.el
index 6b17e861dda..d3f132dae70 100644
--- a/lisp/vc/vc-hg.el
+++ b/lisp/vc/vc-hg.el
@@ -1104,15 +1104,42 @@ hg binary."
   (vc-hg-command nil 0 file "forget"))
 
 (declare-function log-edit-extract-headers "log-edit" (headers string))
+(declare-function log-edit-mode "log-edit" ())
+(declare-function log-edit--toggle-amend "log-edit" (last-msg-fn))
+
+(defun vc-hg-log-edit-toggle-amend ()
+  "Toggle whether this will amend the previous commit.
+If toggling on, also insert its message into the buffer."
+  (interactive)
+  (log-edit--toggle-amend
+   (lambda ()
+     (with-output-to-string
+       (vc-hg-command
+        standard-output 1 nil
+        "log" "--limit=1" "--template" "{desc}")))))
+
+(defvar vc-hg-log-edit-mode-map
+  (let ((map (make-sparse-keymap "Hg-Log-Edit")))
+    (define-key map "\C-c\C-e" 'vc-hg-log-edit-toggle-amend)
+    map))
+
+(define-derived-mode vc-hg-log-edit-mode log-edit-mode "Log-Edit/hg"
+  "Major mode for editing Hg log messages.
+It is based on `log-edit-mode', and has Hg-specific extensions.")
 
 (defun vc-hg-checkin (files comment &optional _rev)
   "Hg-specific version of `vc-backend-checkin'.
 REV is ignored."
-  (apply 'vc-hg-command nil 0 files
-         (nconc (list "commit" "-m")
-                (log-edit-extract-headers '(("Author" . "--user")
-					    ("Date" . "--date"))
-                                          comment))))
+  (let ((amend-extract-fn
+         (lambda (value)
+           (when (equal value "yes")
+             (list "--amend")))))
+    (apply 'vc-hg-command nil 0 files
+           (nconc (list "commit" "-m")
+                  (log-edit-extract-headers `(("Author" . "--user")
+                                              ("Date" . "--date")
+                                              ("Amend" . ,amend-extract-fn))
+                                            comment)))))
 
 (defun vc-hg-find-revision (file rev buffer)
   (let ((coding-system-for-read 'binary)