shortlog: ignore commits with missing authors

Most of git's traversals are robust against minor breakages
in commit data. For example, "git log" will still output an
entry for a commit that has a broken encoding or missing
author, and will not abort the whole operation.

Shortlog, on the other hand, will die as soon as it sees a
commit without an author, meaning that a repository with
a broken commit cannot get any shortlog output at all.

Let's downgrade this fatal error to a warning, and continue
the operation.

We simply ignore the commit and do not count it in the total
(since we do not have any author under which to file it).
Alternatively, we could output some kind of "<empty>" record
to collect these bogus commits. It is probably not worth it,
though; we have already warned to stderr, so the user is
aware that such bogosities exist, and any placeholder we
came up with would either be syntactically invalid, or would
potentially conflict with real data.

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