t: introduce GIT_TEST_DEFAULT_REF_FORMAT envvar
Introduce a new GIT_TEST_DEFAULT_REF_FORMAT environment variable that
lets developers run the test suite with a different default ref format
without impacting the ref format used by non-test Git invocations. This
is modeled after GIT_TEST_DEFAULT_OBJECT_FORMAT, which does the same
thing for the repository's object format.
Adapt the setup of the `REFFILES` test prerequisite to be conditionally
set based on the default ref format.
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>