Allow users to efficiently lookup consecutive entries that are expected
to be found on the same revindex by exporting `find_revindex_position`:
this function takes a pointer to revindex itself, instead of looking up
the proper revindex for a given packfile on each call.
Signed-off-by: Vicent Marti <tanoku@gmail.com> Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>