fetch: don't try to update unfetched tracking refs

Since commit f269048 (fetch: opportunistically update tracking refs,
2013-05-11) we update tracking refs opportunistically when fetching
remote branches.  However, if there is a configured non-pattern refspec
that does not match any of the refspecs given on the command line then a
fatal error occurs.

Fix this by setting the "missing_ok" flag when calling get_fetch_map.

Test-added-by: Jeff King <peff@peff.net>
Signed-off-by: John Keeping <john@keeping.me.uk>
Acked-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>