[markdown] fix: mmm clobbering comment-start in Markdown Git commit

Fixes #14195.

CHANGELOG.develop

@@ -2596,6 +2596,8 @@ files (thanks to Daniel Nicolai)
 - Improvements:
   - Evilfied =dap= debug windows
 **** Markdown
+- Disable =mmm-mode= in Git commit buffers, in order to allow the use of Markdown
+  mode when committing (thanks to Keith Pinson)
 - New layer variable =markdown-mmm-auto-modes= which is a list of language names
   or lists of language and mode names that are supported in source blocks, you
   can add you own modes, see layer README.org for more info

layers/+lang/markdown/funcs.el

@@ -10,8 +10,9 @@
 ;;; License: GPLv3
 
 (defun spacemacs/activate-mmm-mode ()
+  (unless git-commit-mode
   ;; Enable `mmm-mode'.
-  (mmm-mode 1))
+    (mmm-mode 1)))
 
 ;; stolen from http://stackoverflow.com/a/26297700
 ;; makes markdown tables saner via orgtbl-mode