git: Accept commits and tags with malformed signatures

We always check objects when either fetching or receiving objects via git-fsck(1). While this guards us against known-malicious and severely malformed objects, a subset of the checks simply point out issues which aren't severe. We have thus already disabled checks for commits with a bad timezone to enable mirroring of such repositories as well as pushing legacy history containing them into a Gitaly-hosted repository.

Another benign issue which both Git and Gitaly can handle just fine are commits and tags which lack a timestamp in either their committer or author fields. Given that we have added a few tests in the preceding commits which assert we can handle them, let's disable these checks.

Fixes #3687 (closed)