git fetch-pack: do not complain about "no common commits" in an empty repo

git fetch-pack: do not complain about "no common commits" in an empty repo

If the repo is empty, it is obvious that there are no common commits
when fetching from _anywhere_.

So there is no use in saying it in that case, and it can even be
annoying.  Therefore suppress the message unilaterally if the repository
is empty prior to the fetch.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>