git-commit-mode: Add kill-buffer-hook to abort.

This moves the core functionality of `git-commit-abort' to
`git-commit--kill-buffer'.  It is actually added to
`kill-buffer-query-functions' as a workaround to be called before
`server-kill-buffer' is called.

This makes sure that killing the buffer will send an abort message
"-error Commit aborted by user" to git and properly aborts the commit.

* git-commit-mode.el (git-commit-commit): Make sure
  `git-commit--kill-buffer' is not called after commit.
  (git-commit--kill-buffer): New function.
  (git-commit-abort): Rely on `kill-buffer' to abort.
  (git-commit-mode): Add `git-commit--kill-buffer' hook.

Signed-off-by: Rüdiger Sonderfeld <ruediger@c-plusplus.de>