Make handleRepoUpdate block when cloning

Created by: nicksnyder

handleRepoUpdate is documented to be synchronous but the current behavior in the clone case is non-blocking. This renders the rate limiter in repoupdater ineffective when repos are being cloned. Fortunately, I don't think this has been causing many issues in practice because gitserver still enforces GitMaxConcurrentClone per replica.

Consider this PR a bug report with a suggested fix, but I have not tested it and I don't want to own merging this.