magit-gitk-executable: locate executable at load-time

Locate the gitk executable at load-time when setting the default
value of `magit-gitk-executable', instead of doing it every time
`magit-run-gitk' is used.  This builds on a similar hack used
when setting the default value of `magit-git-executable'.

magit.el

@@ -264,9 +264,15 @@ tramp to connect to servers with ancient Git versions."
   :group 'magit-process
   :type '(repeat string))
 
-(defcustom magit-gitk-executable (executable-find "gitk")
+(defcustom magit-gitk-executable
+  (or (eq system-type 'windows-nt)
+      (let ((exe (expand-file-name
+                  "gitk" (file-name-nondirectory magit-git-executable))))
+        (and (file-executable-p exe) exe))
+      (executable-find "gitk") "gitk")
   "The Gitk executable."
   :group 'magit-process
+  :set-after '(magit-git-executable)
   :type 'string)
 
 (defcustom magit-emacsclient-executable
@@ -7590,38 +7596,8 @@ blame to center around the line point is on."
 Without a prefix argument run `gitk --all', with
 a prefix argument run gitk without any arguments."
   (interactive "P")
-  (let ((default-directory (magit-get-top-dir)))
-    (cond
-     ((eq system-type 'windows-nt)
-      ;; Gitk is a shell script, and Windows doesn't know how to
-      ;; "execute" it.  The Windows version of Git comes with an
-      ;; implementation of "sh" and everything else it needs, but
-      ;; Windows users might not have added the directory where it's
-      ;; installed to their path
-      (let* ((git-bin-dir
-             ;; According to #824, when using stand-alone installation
-             ;; Gitk maybe installed in ...cmd or ...bin; while Sh
-             ;; is installed in ...bin.
-             (expand-file-name "bin"
-                               (file-name-directory
-                                (directory-file-name
-                                 (file-name-directory
-                                  magit-gitk-executable)))))
-            ;; Adding it onto the end so that anything the user
-            ;; specified will get tried first.  Emacs looks in
-            ;; exec-path; PATH is the environment variable inherited by
-            ;; the process.  I need to change both.
-            (exec-path (append exec-path (list git-bin-dir)))
-            (process-environment process-environment))
-        (setenv "PATH"
-                (format "%s;%s"
-                        (getenv "PATH")
-                        (replace-regexp-in-string "/" "\\\\" git-bin-dir)))
-        (apply #'start-file-process "Gitk" nil "sh" magit-gitk-executable
-               (if arg nil (list "--all")))))
-     (t
-      (apply #'start-file-process "Gitk" nil "sh" magit-gitk-executable
-             (if arg nil (list "--all")))))))
+  (apply #'start-file-process "Gitk" nil "sh" magit-gitk-executable
+         (if arg nil (list "--all"))))
 
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