t: introduce DEFAULT_REPO_FORMAT prereq

t: introduce DEFAULT_REPO_FORMAT prereq

A limited number of tests require repositories to have the default
repository format or otherwise they would fail to run, e.g. because they
fail to detect the correct hash function. While the hash function is the
only extension right now that creates problems like this, we are about
to add a second extension for the ref format.

Introduce a new DEFAULT_REPO_FORMAT prereq that can easily be amended
whenever we add new format extensions. Next to making any such changes
easier on us, the prerequisite's name should also help to clarify the
intent better.

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