use parse_commit_or_die instead of custom message

use parse_commit_or_die instead of custom message

Many calls to parse_commit detect errors and die. In some
cases, the custom error messages are more useful than what
parse_commit_or_die could produce, because they give some
context, like which ref the commit came from. Some, however,
just say "invalid commit". Let's convert the latter to use
parse_commit_or_die; its message is slightly more informative,
and it makes the error more consistent throughout git.

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