stash: add tests to ensure reflog --rewrite --updatref behavior

stash: add tests to ensure reflog --rewrite --updatref behavior

There is missing test coverage to ensure that the resulting reflogs
after a git stash drop has had its old oid rewritten if applicable, and
if the refs/stash has been updated if applicable.

Add two tests that verify both of these happen.

Helped-by: Ævar Arnfjörð Bjarmason <redacted>
Signed-off-by: John Cai <redacted>
Signed-off-by: Junio C Hamano <redacted>