git-commit.sh: Shell script cleanup
This moves "shift" out of the argument processing "case". It also
replaces quite a bit of expr calls with ${parameter#word} constructs,
and uses ${parameter:+word} for avoiding conditionals where possible.
Signed-off-by: David Kastrup <dak@gnu.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>