fix(vc-gutter): use global-diff-hl-mode

Our former approach would enable diff-hl unconditionally. This way, the
`diff-hl-global-modes` variable is respected.

Ref: https://discourse.doomemacs.org/t/4710/2

modules/ui/vc-gutter/config.el

@@ -60,9 +60,9 @@
 ;;; diff-hl
 
 (use-package! diff-hl
-  :hook (find-file    . diff-hl-mode)
-  :hook (vc-dir-mode  . diff-hl-dir-mode)
   :hook (dired-mode   . diff-hl-dired-mode)
+  :hook (doom-first-file . global-diff-hl-mode)
+  :hook (vc-dir-mode . turn-on-diff-hl-mode)
   :hook (diff-hl-mode . diff-hl-flydiff-mode)
   :commands diff-hl-stage-current-hunk diff-hl-revert-hunk diff-hl-next-hunk diff-hl-previous-hunk
   :config