I ran into a problem where I tried updating to a different branch, and the
process appeared to hang.  It turned out that the subrepo revision wasn't
available locally, and I must have originally cloned it from an `hg serve -S` on
a machine that currently wasn't serving anything.  It took 2+ minutes to
timeout, and didn't mention what it was connecting to even then.

There are a couple of other issues in this scenario too.

  - The repo is dirty after the failed checkout because the top level repo is
    updated first.  We should probably make 2 passes- top down to pull
    everything needed, and then do an update once everything is in place.

  - Something must be reading .hgsubstate from wdir because if the same merge
    command is run after the timeout, a prompt is issued that the local and
    remote subrepo diverged, instead of hanging.  But it lists the local version
    and remote version as having the same hash.