sync to/from remote locations with tree doesn't work

[Michael Gliwinski]

Trying to colo-sync-from / colo-sync-to remote locations which have a working tree stops with "Not a branch" error. E.g.:

$ # on a remote host
$ bzr colo-init /tmp/colo-rwt-test-remote

$ # on local host
$ bzr colo-init /tmp/colo-rwt-test-local
$ cd /tmp/colo-rwt-test-local
$ colo-sync-to sftp://remote.host//tmp/colo-rwt-test-remote
bzr: ERROR: Not a branch: "/tmp/colo-rwt-test-remote/.bzr/branches/trunk/".

What's happening is `ColocatedWorkspace().repo_location` is set to a file:// URL because `BzrDir.open('sftp://.../colo-workspace').open_branch().base.startswith('file:')`.