shortlog: don't warn on empty author

shortlog: don't warn on empty author

Git tries to avoid creating a commit with an empty author
name or email. However, commits created by older, less
strict versions of git may still be in the history.  There's
not much point in issuing a warning to stderr for an empty
author. The user can't do anything about it now, and we are
better off to simply include it in the shortlog output as an
empty name/email, and let the caller process it however they
see fit.

Older versions of shortlog differentiated between "author
header not present" (which complained) and "author
name/email are blank" (which included the empty ident in the
output).  But since switching to format_commit_message, we
complain to stderr about either case (linux.git has a blank
author deep in its history which triggers this).

We could try to restore the older behavior (complaining only
about the missing header), but in retrospect, there's not
much point in differentiating these cases. A missing
author header is bogus, but as for the "blank" case, the
only useful behavior is to add it to the "empty name"
collection.

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