Prepare testsuite for a "git clone" that packs refs

Prepare testsuite for a "git clone" that packs refs

t5515-fetch-merge-logic removes many, but not all, refs between each test.
This is done by removing the corresponding refs/foo/* files in the .git/refs
hierarchy. However, once "git clone" starts producing packed refs, these refs
will no longer be in the .git/refs hierarchy, but rather listed in
.git/packed-refs. This patch teaches t5515-fetch-merge-logic to remove the
refs using "git update-ref -d" which properly handles packed refs.

Signed-off-by: Johan Herland <johan@herland.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>