Reenable upgrade to Git v2.28.0

This reverts commit b3c31ec9, reversing changes made to 8d004a2a.

A patch to fix the root cause has been sent upstream to git-core. Waiting for this would block a few weeks until Git v2.28.1 is released, and reverting all the work toward v2.28.0 would be a lot of effort. So we've removed the reference-transaction hook in Gitaly for now in gitaly!2454 (merged) to unblock the upgrade and fix the regression discovered in gitlab#233964 (closed).

A testcase has been added in (gitlab-org/quality/testcases#962 (closed)) to catch the regression.