Repo language detected no more that once a week

Noted by analyzing Sidekiq usage patterns1, the commit languages topped the charts in time spend. Most, almost all, of this time is spend on Gitaly.

Now Gitaly could reject the call with a FailedPrecondition if it finds it's scanning the repository too much. The heuristic to be used for this would be cache modification time. The cache will be updated each linguist run, and only RPC per repository is called at once from the client side.