t0001: exercise initialization with ref formats more thoroughly

t0001: exercise initialization with ref formats more thoroughly

While our object format tests for git-init(1) exercise tests with all
known formats in t0001, the tests for the ref format don't. This leads
to some missing test coverage for interesting cases, like whether or not
a non-default ref storage format causes us to bump the repository format
version. We also don't test for the precedence of the `--ref-format=`
and the `GIT_DEFAULT_REF_FORMAT=` environment variable.

Extend the test suite to cover more scenarios related to the ref format.

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