testing: add something to help reproduce issues with lots of repos

Created by: ijt

To reproduce customer issues involving many repos, it would be useful to have a service that would dynamically create and serve any number of repos, along with config files to paste into a sourcegraph instance. Let's call it fakehub for now.

For example, here's a command that could serve 1000 copies of a git repo:

$ fakehub --addr=:8090 -n 1000 path/to/some/git/repo

The copies could be accessed like this:

git clone http://localhost:8090/repo/1

A config file could be dynamically generated at

http://localhost:8090/sgconfig.json

for easy pasting into a sourcegraph instance.