Fix font-locking of action and file

This patch changes the regular expression that is supposed to match the
lines containing "modified:" or "some-file.txt" in the example below. It
is desirable that this regexp does not match multiple lines.

    #       modified:   git-commit-mode.el
    #
    # Untracked files:
    #       some-file.txt

The reason is that if the commit message contains a diff (i.e. "git
commit -v" was run) the "[^:]:" regexp can match multiple lines if there
is a colon in the diff. In the example above, the match would span from
'some-file.txt' up to the colon in the diff. This results in overriding
the font-locking in the diff.

Signed-off-by: Michal Sojka <sojkam1@fel.cvut.cz>