UserSquash, the first repository mutation done by HGitaly

Here, the main challenge was to accept all necessary information that has to be given back to the Rails app in pre/post-receive hooks. Actually this is performed from repo config and environment variables by the lower layer: heptapod/py-heptapod>. So we need to perform the settings expected by the latter.

The actual method itself is almost a pretense – the infrastructure is what matters most.