t1301: mark test for `core.sharedRepository` as reffiles specific

t1301: mark test for `core.sharedRepository` as reffiles specific

In t1301 we verify whether reflog files written by the "files" ref
backend correctly honor permissions when "core.sharedRepository" is set.
The test logic is thus specific to the reffiles backend and will not
work with any other backends.

Mark the test accordingly with the REFFILES prereq.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Reviewed-by: Christian Couder <christian.couder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>