parse_commit(): describe more date-parsing failure modes

parse_commit(): describe more date-parsing failure modes

The previous few commits improved the parsing of dates in malformed
commit objects. But there's one big case left implicit: we may still
feed garbage to parse_timestamp(). This is preferable to trying to be
more strict, but let's document the thinking in a comment.

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