I configured git so that it uses emacsclient when editing commit messages. The only problem is that these messages are edited in
fundamental-mode. Which hook should I use to have these commit messages buffer in
.git/COMMIT_EDITMSG as the temporary file for the commit message. Since Emacs will be prompted to edit it by git, you can associate that file with
markdown-mode by adding something like
(add-to-list 'auto-mode-alist '("COMMIT_EDITMSG" . markdown-mode))
to your init file.
I believe this will do what your after, but i have not tested personally
(setq git-commit-major-mode 'markdown-mode)
Looking at the code in the link below its set with
defcustom to allow you to override to any mode you like.