Add mode hooks to {git-commit,gitconfig,gitignore}-mode

I only actually needed this for the git-commit-mode, because I wanted to
turn off the annoying auto-fill, I prefer to do it myself, and sometimes
you want long lines for e.g. quoting log output.

So now you can do that like:

    (add-hook 'git-commit-mode-hook
              '(lambda ()
                 (message "Running the git commit mode hook")
                 (turn-off-auto-fill)))

I didn't add a git-rebase-mode-hook because it's already here in some
form, although I don't understand how it's supposed to work:

    $ git --no-pager grep git-rebase-mode-hook
    git-rebase-mode.el:362:(add-hook 'git-rebase-mode-hook 'git-rebase-mode-show-keybindings t)
    git-rebase-mode.el:367:(add-hook 'git-rebase-mode-hook 'git-rebase-mode-disable-before-save-hook)