gitserver: Run repoInfo in parallel

Created by: ryanslade

This speeds up calls with a large number of repos as each call needs to make several requests to git so we spend a lot of time waiting on IO.

I would prefer to do some caching too, but I'm not sure how to go about ensuring that the cache is invalidated correctly.

Part of: https://github.com/sourcegraph/sourcegraph/issues/8725