Reference-transaction hook broken on interleaving hook calls

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.