magit-wip-mode: Speedup activation by avoiding duplicated work

Reduce work to one call to `magit-toplevel' for each directory
from which one or more files are being visited, and to one call
to `magit-tracked-files' for each repository from which one or
more files are being visited.

Unfortunately we cannot do the same if `magit-wip-after-save-mode'
is enabled directly, but that is only provided as a separate mode
for backward compatibility anyway.

Closes #4768.

Author: tarsius

lisp/magit-wip.el

@@ -103,6 +103,8 @@ is used as `branch-ref'."
 
 ;;; Modes
 
+(defvar magit--wip-activation-cache nil)
+
 ;;;###autoload
 (define-minor-mode magit-wip-mode
   "Save uncommitted changes to work-in-progress refs.
@@ -119,7 +121,8 @@ but that is discouraged."
   :lighter magit-wip-mode-lighter
   :global t
   (let ((arg (if magit-wip-mode 1 -1)))
-    (magit-wip-after-save-mode arg)
+    (let ((magit--wip-activation-cache (list t)))
+      (magit-wip-after-save-mode arg))
     (magit-wip-after-apply-mode arg)
     (magit-wip-before-change-mode arg)
     (magit-wip-initial-backup-mode arg)))
@@ -144,8 +147,30 @@ variant `magit-wip-after-save-mode'."
 
 (defun magit-wip-after-save-local-mode-turn-on ()
   (when (and buffer-file-name
-             (magit-inside-worktree-p t)
-             (magit-file-tracked-p buffer-file-name))
+             (if magit--wip-activation-cache
+                 (if-let ((elt (assoc default-directory
+                                      magit--wip-activation-cache)))
+                     (and-let* ((top (cadr elt)))
+                       (member (file-relative-name buffer-file-name top)
+                               (cddr elt)))
+                   (if-let ((top (magit-toplevel)))
+                       (let (files)
+                         (if-let ((elt (assoc top magit--wip-activation-cache)))
+                             (setq files (cddr elt))
+                           (progn
+                             (setq files (let ((default-directory top))
+                                           (magit-tracked-files)))
+                             (push `(,top ,top ,@files)
+                                   magit--wip-activation-cache)
+                             (unless (eq default-directory top)
+                               (push `(,default-directory ,top ,@files)
+                                     magit--wip-activation-cache))))
+                         (member (file-relative-name buffer-file-name) files))
+                     (push (list default-directory nil)
+                           magit--wip-activation-cache)
+                     nil))
+               (and (magit-inside-worktree-p t)
+                    (magit-file-tracked-p buffer-file-name))))
     (magit-wip-after-save-local-mode)))
 
 ;;;###autoload