Disable automatic git maintenance, too

Git 2.30 (released in 2020) introduced the new maintenance.auto config option that avoids [1] extra process invocation that would previously happen even with gc.auto disabled (which we already disable).

This has recently [2] been changed so that disabling gc.auto has the same effect, but to let the users of older Git versions benefit from this feature, disable maintenance.auto explicitly as well. Git versions that don’t know about the maintenance.auto option (< 2.30) will not mind if we set this option since it will simply be ignored.