fetch_refs_via_pack: call report_unmatched_refs

fetch_refs_via_pack: call report_unmatched_refs

"git fetch" currently doesn't bother to check that it got all refs it
sought, because the common case of requesting a nonexistent ref triggers
a die() in get_fetch_map.  However, there's at least one case that
slipped through: "git fetch REMOTE SHA1" if the server doesn't allow
requests for unadvertised objects.  Make fetch_refs_via_pack (which is
on the "git fetch" code path) call report_unmatched_refs so that we at
least get an error message in that case.

Signed-off-by: Matt McCutchen <matt@mattmccutchen.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>