fetch: use oidset to keep the want OIDs for faster lookup

fetch: use oidset to keep the want OIDs for faster lookup

During git-fetch, the client checks if the advertised tags' OIDs are
already in the fetch request's want OID set. This check is done in a
linear scan. For a repository that has a lot of refs, repeating this
scan takes 15+ minutes. In order to speed this up, create a oid_set for
other refs' OIDs.

Signed-off-by: Masaya Suzuki <masayasuzuki@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>