sequencer: use a helper to find the commit message

sequencer: use a helper to find the commit message

It is actually not safe to look for a commit message by looking for the
first empty line and skipping it.

The find_commit_subject() function looks more carefully, so let's use
it. Since we are interested in the entire commit message, we re-compute
the string length after verifying that the commit subject is not empty
(in which case the entire commit message would be empty, something that
should not happen but that we want to handle gracefully).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>