reftable: JGit compatibility tests

While the reftable backend is a recent addition to Git, it has been part of JGit since 2017 already. Given that there are essentially two different implementations of the reftable format there is a very real risk of these two diverge and become incompatible with each other, which would be a shame.

Extend the test suite to have some basic compatibility tests between the Git and JGit implementations of reftables.

Closes reftable: compatibility tests with JGit (#292 - closed).