git-commit: clean up die messages

These are three types of cleanups here:

1. remove newline from die message (die/report adds it
already)
2. typo: s/merger/merge/
3. the old "* no commit message?  aborting commit." is now
prepended with "fatal: ", making the asterisk look a
little funny. Let's just remove it.

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