filter_refs(): delete matched refs from sought list

filter_refs(): delete matched refs from sought list

Remove any references that are available from the remote from the
sought list (rather than overwriting their names with NUL characters,
as previously).  Mark matching entries by writing a non-NULL pointer
to string_list_item::util during the iteration, then use
filter_string_list() later to filter out the entries that have been
marked.

Document this aspect of fetch_pack() in a comment in the header file.
(More documentation is obviously still needed.)

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