refs: fix interleaving hook calls with reference-transaction hook

In order to not repeatedly search for the reference-transaction hook in
case it's getting called multiple times, we use a caching mechanism to
only call `find_hook()` once. What was missed though is that the return
value of `find_hook()` actually comes from a static strbuf, which means
it will get overwritten when calling `find_hook()` again. As a result,
we may call the wrong hook with parameters of the reference-transaction
hook.

This scenario was spotted in the wild when executing a git-push(1) with
multiple references, where there are interleaving calls to both the
update and the reference-transaction hook. While initial calls to the
reference-transaction hook work as expected, it will stop working after
the next invocation of the update hook. The result is that we now start
calling the update hook with parameters and stdin of the
reference-transaction hook.

This commit fixes the issue by storing a copy of `find_hook()`'s return
value in the cache.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

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