gitserver: Abort doBackgroundRepoUpdate when rate limiter is zero

Created by: ryanslade

If we don't abort then the background job can potentially block forever if a zero rate limit has been set. This would leak goroutines and hold a cloneLimiter lock.

We do not add a timeout as it's hard to pick on as this is a legitimate background process that could take a long time to complete for large repos.

Closes: https://github.com/sourcegraph/sourcegraph/issues/19720