setup: use "reftable" format when experimental features are enabled

setup: use "reftable" format when experimental features are enabled

With the preceding commit we have announced the switch to the "reftable"
format in Git 3.0 for newly created repositories. The format is being
battle tested by GitLab and a couple of other developers, and except for
a small handful of issues exposed early after it has been merged it has
been rock solid. Regardless of that though the test user base is still
comparatively small, which increases the risk that we miss critical
bugs.

Address this by enabling the reftable format when experimental features
are enabled. This should increase the test user base by some margin and
thus give us more input before making the format the default.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>