Transactional voting via reference-transaction hook

The reference-transaction hook included in Git v2.28.0 is currently broken with interleaving hook calls, where the wrong hook would start to be executed with parameters of the reference-transaction hook. As a result, the commit introducing it got reverted.

At the point of writing, there is no release of Git including the fixed version of that hook yet, and chances are good we'll have to wait another two months until Git v2.29.0. To unblock ourselves, a workaround has been added in c9d0ae5a (gitaly-hooks: Fixup reference-transaction hook name based on arguments, 2020-08-31). Together with the testcase in this commit's predecessor, we can now include the reference-transaction hook again and be sure to not trigger the original issue anymore.

This reverts commit 19e2caa3 and wires up the reference-transaction hook again.