t1401: stop treating FETCH_HEAD as real reference
One of the tests in t1401 asserts that we can create a symref from a
symbolic reference to a top-level reference, which is done by linking
from `refs/heads/top-level` to `FETCH_HEAD`. But `FETCH_HEAD` is not a
proper reference and doesn't even follow the loose reference format, so
it is not a good candidate for the logic under test.
Refactor the test to use `ORIG_HEAD` instead of `FETCH_HEAD`. This also
works with other backends than the reffiles one.
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>