filter_refs(): do not check the same sought_pos twice

filter_refs(): do not check the same sought_pos twice

Once a match has been found at sought_pos, the entry is zeroed and no
future attempts will match that entry.  So increment sought_pos to
avoid checking against the zeroed-out entry during the next iteration.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>