bug#29366: gitmerge to handle NEWS better

[Glenn Morris <rgm@gnu.org>]

Glenn Morris wrote:

> Currently etc/NEWS in the emacs-26 branch has been renamed to
> etc/NEWS.26 in master. Running M-x gitmerge, this is not detected.
> emacs-26:etc/NEWS changes are merged to master:etc/NEWS, resulting in a
> merge conflict every time NEWS is touched in emacs-26.
> Can gitmerge do something to help here?

I added some code for this in 0b6f4f2 that I thought worked.
There's pre-existing gitmerge code that uses smerge to try and
auto-resolve conflicts. I just added a special-case for NEWS at the same
place.

However, it's missing a step that will automatically continue the merge
(ie, do the equivalent of running M-x gitmerge after manually fixing a
conflict). I cannot see how this would work for the "smerge fixed a
conflict automatically" case either. I don't know how to simulate an
smerge-fixable conflict to test.

Am I overlooking something, or is the code missing something like the
following:

--- a/admin/gitmerge.el
+++ b/admin/gitmerge.el
@@ -431,7 +431,8 @@ gitmerge-resolve-unmerged
 	      (setq conflicted t)
 	    ;; Mark as resolved
 	    (call-process "git" nil t nil "add" file)))
-	(when conflicted
+	(if (not conflicted)
+	    (if files (gitmerge-maybe-resume 'noask))
 	  (with-current-buffer (get-buffer-create gitmerge-warning-buffer)
 	    (erase-buffer)
 	    (insert "For the following files, conflicts could\n"
@@ -457,7 +458,7 @@ gitmerge-repo-clean
 		    "diff" "--name-only")
       (zerop (buffer-size))))
 
-(defun gitmerge-maybe-resume ()
+(defun gitmerge-maybe-resume (&optional noask)
   "Check if we have to resume a merge.
 If so, add no longer conflicted files and commit."
   (let ((mergehead (file-exists-p
@@ -469,7 +470,7 @@ gitmerge-maybe-resume
 	       (not (gitmerge-repo-clean)))
       (user-error "Repository is not clean"))
     (when statusexist
-      (if (not (y-or-n-p "Resume merge? "))
+      (if (not (or noask (y-or-n-p "Resume merge? ")))
 	  (progn
 	    (delete-file gitmerge-status-file)
 	    ;; No resume.