git/stats: Adapt tests to Git enabling reverse indices by default

With 849c8b3dbf (Merge branch 'tb/pack-revindex-on-disk', 2023-04-27), Git is about to enable reverse indices by default. This is nothing that greatly impacts us as we have enabled writing reverse indices starting with c03d2678 (git: Inject pack.writeReverseIndex into commands generating packfiles, 2021-04-01) already.

That being said, some of the tests for repository statistics depend on reverse indices not being enabled by default as we run git-repack(1) manually there and then check whether reverse indices have been written or not. These tests thus break with the upstream change.

Adapt the tests to explicitly enable writing reverse indices so that they succeed with and without the upstream change.