Fix the byte-compiler warning in a different way

Since (magit-gitdir "hooks/commit-msg") returns nil instead
of (magit-git-dir "hooks/commit-msg"), which returns the expected
path, a working fix is to call magit-gitdir without any args and
concat it with the hooks/commit-msg.

Change-Id: I58a185bcc4805bf962372935be562843c778414c

gerrit.el

@@ -336,7 +336,8 @@ This refspec is a string of the form \='refs/changes/xx/xx/x\='."
 
 (defun gerrit--ensure-commit-msg-hook-exists ()
   "Create a commit-msg hook, if it doesn't exist."
-  (let ((hook-file (magit-git-dir "hooks/commit-msg")))
+  (let ((hook-file
+         (concat (magit-gitdir) (convert-standard-filename "hooks/commit-msg"))))
     (unless (file-exists-p hook-file)
       (message "downloading commit-msg hook file")
       (url-copy-file