Disable company-mode during git commits

Why didn't I configure this earlier? For years, my workflow involved checking a
buffer's major mode and then extending that major-mode's hook. Confusingly (to
me), the `major-mode` for `COMMIT_EDITMSG` is `text-mode`, and I didn't want to
disable `company-mode` for *all* `text-mode` buffers, which is what the
following would have done:

```elisp
(add-hook 'text-mode-hook (lambda () (company-mode -1))
```

Thankfully I recently invested some time into learning more about Emacs's
offline help system, `Info-mode`, so -- putting that knowledge to work -- I ran
`info-apropos` and searched "magit commit". After ~5 minutes of reading I knew
the recommended way of configuring this was to modify `git-commit-setup-hook`.

How validating!

emacs/.emacs.d/wpc/wpc-misc.el

@@ -127,6 +127,7 @@
 ;; git integration
 (use-package magit
   :config
+  (add-hook 'git-commit-setup-hook (lambda () (company-mode -1)))
   (setq magit-display-buffer-function
         #'magit-display-buffer-fullframe-status-v1))