Add new user option 'diff-add-log-use-relative-names'

* .dir-locals.el: Set new option to t
* etc/NEWS: Mention new option.
* lisp/vc/diff-mode.el (diff-add-log-use-relative-names): Add new option.
(diff-add-log-current-defuns): Use new option.
* doc/emacs/maintaining.texi (Log Buffer): Mention new option.

.dir-locals.el

@@ -5,7 +5,8 @@
          (sentence-end-double-space . t)
          (fill-column . 70)
 	 (emacs-lisp-docstring-fill-column . 65)
-         (bug-reference-url-format . "https://debbugs.gnu.org/%s")))
+         (bug-reference-url-format . "https://debbugs.gnu.org/%s")
+	 (diff-add-log-use-relative-names . t)))
  (c-mode . ((c-file-style . "GNU")
             (c-noise-macro-names . ("INLINE" "ATTRIBUTE_NO_SANITIZE_UNDEFINED" "UNINIT" "CALLBACK" "ALIGN_STACK"))
             (electric-quote-comment . nil)

doc/emacs/maintaining.texi

@@ -690,11 +690,15 @@ started editing (@pxref{Old Revisions}), type @kbd{C-c C-d}
 
 @kindex C-c C-w @r{(Log Edit mode)}
 @findex log-edit-generate-changelog-from-diff
+@vindex diff-add-log-use-relative-names
   To help generate ChangeLog entries, type @kbd{C-c C-w}
 (@code{log-edit-generate-changelog-from-diff}), to generate skeleton
 ChangeLog entries, listing all changed file and function names based
 on the diff of the VC fileset.  Consecutive entries left empty will be
-combined by @kbd{C-q} (@code{fill-paragraph}).
+combined by @kbd{C-q} (@code{fill-paragraph}).  By default the
+skeleton will just include the file name, without any leading
+directories.  If you wish to prepend the leading directories up to the
+VC root, customize @code{diff-add-log-use-relative-names}.
 
 @kindex C-c C-a @r{(Log Edit mode)}
 @findex log-edit-insert-changelog

etc/NEWS

@@ -1340,6 +1340,11 @@ Sets the value of the buffer-local variable 'whitespace-style' in
 'diff-mode' buffers.  By default, this variable is '(face trailing)',
 which preserves behavior from previous Emacs versions.
 
++++
+*** New user option 'diff-add-log-use-relative-names'.
+If non-nil insert file names in ChangeLog skeletons relative to the
+VC root directory.
+
 ** Ispell
 
 ---

lisp/vc/diff-mode.el

@@ -2336,10 +2336,21 @@ Call FUN with two args (BEG and END) for each hunk."
   (let ((inhibit-read-only t))
     (undo arg)))
 
+(defcustom diff-add-log-use-relative-names nil
+  "Use relative file names when generating ChangeLog skeletons.
+The files will be relative to the root directory of the VC
+repository.  This option affects the behaviour of
+`diff-add-log-current-defuns'."
+  :type 'boolean
+  :safe #'booleanp
+  :version "29.1")
+
 (defun diff-add-log-current-defuns ()
   "Return an alist of defun names for the current diff.
 The elements of the alist are of the form (FILE . (DEFUN...)),
-where DEFUN... is a list of function names found in FILE."
+where DEFUN... is a list of function names found in FILE.  If
+`diff-add-log-use-relative-names' is non-nil, file names in the alist
+are relative to the root directory of the VC repository."
   (save-excursion
     (goto-char (point-min))
     (let* ((defuns nil)
@@ -2373,7 +2384,12 @@ where DEFUN... is a list of function names found in FILE."
           ;; hunks (e.g., "diff --git ..." etc).
           (re-search-forward diff-hunk-header-re nil t)
         (setq hunk-end (save-excursion (diff-end-of-hunk)))
-        (pcase-let* ((filename (substring-no-properties (diff-find-file-name)))
+        (pcase-let* ((filename (substring-no-properties
+                                (if diff-add-log-use-relative-names
+                                    (file-relative-name
+                                     (diff-find-file-name)
+                                     (vc-root-dir))
+                                  (diff-find-file-name))))
                      (=lines 0)
                      (+lines 0)
                      (-lines 0)