With the same reasoning as for all the other environment variables,
rename the GIT_TEST_DEFAULT_REF_FORMAT environment variable to
GIT_TEST_DEFAULT_REF_STORAGE_FORMAT.
Note that in this case, the old name is not retained for backwards
compatibility as this is only running in the context of tests anyway.
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>