mailmap: do not resolve blobs in a non-repository

mailmap: do not resolve blobs in a non-repository

The mailmap code may be triggered outside of a repository by
git-shortlog. There is no point in looking up a name like
"HEAD:.mailmap" there; without a repository, we have no
refs.

This is unlikely to matter much in practice for the current
code, as we would simply fail to find the ref. But as the
refs code learns about new backends, this is more important;
without a repository, we do not even know which backend to
look at.

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