Disable auto.gc for large repositories

As a follow up to STASH-4963 we should consider turning off Git auto gc for large (for some tbd definition of large) repositories.

Auto GC may be triggered at any time and can take a considerable amount of time for large repositories during which users don't get a hint as to what is going on.

STASH-4963 demonstrates one example where GC is happening during a push.

For forks we already turn off auto gc and do the garbage collection in the background. While this shouldn't be the default for all repositories (auto gc has the advantage of happening when necessary without any scheduling concern from our end) it could be beneficial for teams with large repositories as it would avoid opaque pauses.