t5312: create bogus ref as necessary

Some tests in t5312 create an illegally-named ref, and then see how
various operations handle it. But between those operations, we also do
some more setup (e.g., repacking), and we are subtly depending on how
those setup steps react to the illegal ref.

To future-proof us against those behaviors changing, let's instead
create and clean up our bogus ref on demand in the tests that need it.

This has two small extra advantages:

 - the tests are more stand-alone; we do not need an extra test to clean
   up the ref before moving on to other parts of the script

 - the creation and cleanup is together in one helper function. Because
   these depend on touching the refs in the filesystem directly, they
   may need to be tweaked for a world with alternate backends (they have
   not been noticed so far in the reftable work because with a non-file
   backend the tests don't fail; they simply become uninteresting noops
   because the broken ref isn't read at all).

Signed-off-by: Jeff King <peff@peff.net>
Reviewed-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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