t/perf: add perf test for ref tombstone scenarios

t/perf: add perf test for ref tombstone scenarios

Add performance tests for update-ref when many tombstones are present
in a reftable.

The first test exercises the scenario where all refs are deleted
(creating tombstones) and then re-created with the same names, which
currently exhibits quadratic behavior.

The second test uses a separate repository with an asymmetric variant
where refs are deleted and then new, differently-named refs are
created.  When the tombstones sort after the new refs, every create
scans all tombstones, making this case even worse than re-creating
the same refs.

Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Kristofer Karlsson <krka@spotify.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>