log-edit-generate-changelog-from-diff: Special-case Summary field

* lisp/vc/add-log.el (log-edit-fill-entry): Delete unused
declaration.
* lisp/vc/log-edit.el (log-edit-generate-changelog-from-diff):
Special-case inserting a single line into the Summary field.

lisp/vc/add-log.el

@@ -314,7 +314,6 @@ as a list of strings."
                                ",[[:blank:]]*" t)
            finally do (skip-chars-backward "\n[:blank:]")))
 
-(declare-function log-edit-fill-entry "log-edit")
 (defun change-log-insert-entries (changelogs)
   "Format and insert CHANGELOGS into current buffer.
 CHANGELOGS is a list in the form returned by

lisp/vc/log-edit.el

@@ -1057,7 +1057,7 @@ names into a single entry where they all share the same description.
 Should you need to look at the diffs themselves, they can be found
 in the \"*vc-diff*\" buffer produced by this command."
   (interactive)
-  (change-log-insert-entries
+  (let* ((entries
           (with-current-buffer
               (let* ((diff-buf nil)
                      ;; Unfortunately, `log-edit-show-diff' doesn't have a
@@ -1070,7 +1070,16 @@ in the \"*vc-diff*\" buffer produced by this command."
                         . ((allow-no-window . t)))))
                 (log-edit-show-diff)
                 diff-buf)
-     (diff-add-log-current-defuns))))
+            (diff-add-log-current-defuns)))
+         (single-line-summary
+          (and (length= entries 1)
+               (length< (cdar entries) 2)
+               (< (point) (save-excursion (rfc822-goto-eoh) (point)))
+               (save-excursion (forward-line 0) (looking-at "Summary:")))))
+    (change-log-insert-entries entries)
+    (when single-line-summary
+      (delete-char -1)
+      (insert " "))))
 
 (defun log-edit-insert-changelog (&optional use-first)
   "Insert a VC commit log message by looking at the ChangeLog.