get_fetch_map(): tighten checks on dest refs

The code to check the refname we store the fetched result locally did not bother checking the first 5 bytes of it, presumably assuming that it always begin with "refs/". For a fetch refspec (or the result of applying wildcard on one), we always want the RHS to map to something inside "refs/" hierarchy, so let's spell that rule out in a more explicit way. Signed-off-by: Junio C Hamano <gitster@pobox.com>