Again, the trick is to use the Emacs variable fill-nobreak-predicate, but it's a bit trickier than my earlier example.
Don't want a line to break in emacs even in auto-fill-mode?
The code is very similar to the earlier code:
(defun tkb-magit-commit-fill-nobreak-p ()
"Don't fill on the first line of a magit commit message."
(= 1 (line-number-at-pos)))
(defun tkb-magit-commit-find-file-hook ()
(interactive)
(when (string-match "COMMIT_EDITMSG\\'" (buffer-file-name))
(make-local-variable 'fill-nobreak-predicate)
(add-to-list 'fill-nobreak-predicate #'tkb-magit-commit-fill-nobreak-p)))
(add-to-list 'find-file-hook #'tkb-magit-commit-find-file-hook t)
The problem is that, depending on where the new hook ends up in the list find-file-hook, it may run after the git-commit-setup-check-buffer find file hook. This is a problem because git-commit-setup-check-buffer runs git-commit-setup which runs normal-mode which runs kill-all-local-variables, which wipes out what our code was trying to do. Sigh.
Instead, just specify the APPEND parameter to add-to-list as t:
(add-to-list 'find-file-hook #'tkb-magit-commit-find-file-hook t)
This adds our hook function at the end of the list, hopefully after git-commit-setup-check-buffer.
I've put this in a gist: