t: use git-show-ref(1) to check for ref existence

Convert tests that use `test_path_is_file` and `test_path_is_missing` to
instead use a set of helpers `test_ref_exists` and `test_ref_missing`.
These helpers are implemented via the newly introduced `git show-ref
--exists` command. Thus, we can avoid intimate knowledge of how the ref
backend stores references on disk.

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

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