master 233b00a683a: Don't load `diff-mode` so eagerly (bug#81222)

branch: master
commit 233b00a683a1a92ab693e0ba5d485c19a0eb8c4b
Author: Stefan Monnier <[email protected]>
Commit: Stefan Monnier <[email protected]>

    Don't load `diff-mode` so eagerly (bug#81222)
    
    * .dir-locals.el (diff-add-log-use-relative-names): Move to `diff-mode`.
    
    * lisp/vc/vc-git.el: Don't require `diff-mode`.
    (vc-git-region-history-mode-map): Don't set the keymap parent yet.
    (vc-git-region-history-font-lock): Declare `diff-font-lock-keywords` to
    silence the compiler warning.
    (vc-git-region-history-mode): Require `diff-mode` and set the keymap
    parent here.
---
 .dir-locals.el    |  4 ++--
 lisp/vc/vc-git.el | 20 ++++++++++++--------
 2 files changed, 14 insertions(+), 10 deletions(-)

diff --git a/.dir-locals.el b/.dir-locals.el
index b1123032443..d1cc520e232 100644
--- a/.dir-locals.el
+++ b/.dir-locals.el
@@ -7,7 +7,6 @@
 	 (emacs-lisp-docstring-fill-column . 72)
          (vc-git-annotate-switches . "-w")
          (bug-reference-url-format . "https://debbugs.gnu.org/%s")
-	 (diff-add-log-use-relative-names . t)
          (etags-regen-regexp-alist
           .
           ((("c" "objc") .
@@ -57,7 +56,8 @@
  (change-log-mode . ((add-log-time-zone-rule . t)
 		     (fill-column . 74)
 		     (mode . bug-reference)))
- (diff-mode . ((mode . whitespace)))
+ (diff-mode . ((mode . whitespace)
+	       (diff-add-log-use-relative-names . t)))
  (emacs-lisp-mode . ((indent-tabs-mode . nil)
                      (electric-quote-comment . nil)
                      (electric-quote-string . nil)
diff --git a/lisp/vc/vc-git.el b/lisp/vc/vc-git.el
index b66f13f7268..e68945cda8d 100644
--- a/lisp/vc/vc-git.el
+++ b/lisp/vc/vc-git.el
@@ -2070,12 +2070,8 @@ This requires git 1.8.4 or later, for the \"-L\" option of \"git log\"."
   (vc-git-command buffer 'async nil "log" "-p" ;"--follow" ;FIXME: not supported?
                   (format "-L%d,%d:%s" lfrom lto (file-relative-name file))))
 
-(require 'diff-mode)
-
 (defvar vc-git-region-history-mode-map
-  (let ((map (make-composed-keymap
-              nil (make-composed-keymap
-                   (list diff-mode-map vc-git-log-view-mode-map)))))
+  (let ((map (make-sparse-keymap)))
     map))
 
 (defvar vc-git--log-view-long-font-lock-keywords nil)
@@ -2083,6 +2079,7 @@ This requires git 1.8.4 or later, for the \"-L\" option of \"git log\"."
   '((vc-git-region-history-font-lock)))
 
 (defun vc-git-region-history-font-lock (limit)
+  (defvar diff-font-lock-keywords)
   (let ((in-diff (save-excursion
                    (beginning-of-line)
                    (or (looking-at "^\\(?:diff\\|commit\\)\\>")
@@ -2094,8 +2091,9 @@ This requires git 1.8.4 or later, for the \"-L\" option of \"git log\"."
                                             limit t)
                          (match-beginning 1)
                        limit))))
-          (let ((font-lock-keywords (if in-diff diff-font-lock-keywords
-                                      vc-git--log-view-long-font-lock-keywords)))
+          (let ((font-lock-keywords
+                 (if in-diff diff-font-lock-keywords
+                   vc-git--log-view-long-font-lock-keywords)))
             (font-lock-fontify-keywords-region (point) end))
           (goto-char end)
           (prog1 (< (point) limit)
@@ -2103,8 +2101,14 @@ This requires git 1.8.4 or later, for the \"-L\" option of \"git log\"."
     nil))
 
 (define-derived-mode vc-git-region-history-mode
-    vc-git-log-view-mode "Git-Region-History"
+  vc-git-log-view-mode "Git-Region-History"
   "Major mode to browse Git's \"log -p\" output."
+  (require 'diff-mode)
+  (defvar diff-mode-map)
+  (unless (keymap-parent vc-git-region-history-mode-map)
+    (set-keymap-parent vc-git-region-history-mode-map
+                       (make-composed-keymap
+                        (list diff-mode-map vc-git-log-view-mode-map))))
   (setq-local vc-git--log-view-long-font-lock-keywords
               log-view-font-lock-keywords)
   (setq-local font-lock-defaults