Be verbose when !initial commit

Be verbose when !initial commit

verbose option in git-commit.sh lead us to run git-diff-index, which
needs a commit-ish we are making diff against.  When we are commiting
the fist set, we obviously don't have any commit-ish in the repo.  So
we just skip the git-diff-index run.

It might be possible to produce diff against empty but do we need
that?

Signed-off-by: Yasushi SHOJI <yashi@atmark-techno.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>