t4207: delete replace references via git-update-ref(1)

t4207: delete replace references via git-update-ref(1)

In t4207 we set up a set of replace objects via git-replace(1). Because
these references should not be impacting subsequent tests we also set up
some cleanup logic that deletes the replacement references via a call to
`rm -rf`. This reaches into the internal implementation details of the
reference backend and will thus break when we grow an alternative refdb
implementation.

Refactor the tests to delete the replacement refs via Git commands so
that we become independent of the actual refdb that's in use. As we
don't have a nice way to delete all replacements or all references in a
certain namespace, we opt for a combination of git-for-each-ref(1) and
git-update-ref(1)'s `--stdin` mode.

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