Calling the Mercurial HTTP backend on a Unix Domain socket

Quoting myself one of the changeset descriptions of the original implementation:

Sadly, supporting Unix Domain sockets does not seem to be straightforward: we'd need to define our own roundtripper, as GitLab already seems to do.

To be consistent with other options from upstream, the new option to specify the socket path should be called hgSocket.

The Mercurial backend itself only needs some configuration to listen on a Unix Domain socket, because Gunicorn provides it out of the box.