Compact commit template message

Compact commit template message

We recently let the user know explicitly that an empty
commit message will abort the commit. However, this adds yet
another line to the template; let's rephrase and re-wrap so
that this fits back on two lines.

This patch also makes the "fatal: empty commit message?"
warning a bit less scary, since this is now a "feature"
instead of an error. However, we retain the non-zero exit
status to indicate to callers that nothing was committed.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>