Code review comment for lp:~spiv/bzr/bzr-ssh-homedir-take-3

I've updated the branch with all the review comments so far.

For convenience, here's the NEWS entry that it adds under New Features in 2.1:

"""
* ``bzr+ssh`` and ``bzr`` paths can now be relative to home directories
  specified in the URL. Paths starting with a path segment of ``~`` are
  relative to the home directory of the user running the server, and paths
  starting with ``~user`` are relative to the home directory of the named
  user. For example, for a user "bob" with a home directory of
  ``/home/bob``, these URLs are all equivalent:

  * ``bzr+ssh://bob@host/~/repo``
  * ``bzr+ssh://bob@host/~bob/repo``
  * ``bzr+ssh://bob@host/home/bob/repo``

  If ``bzr serve`` was invoked with a ``--directory`` argument, then no
  home directories outside that directory will be accessible via this
  method.

  This is a feature of ``bzr serve``, so pre-2.1 clients will
  automatically benefit from this feature when ``bzr`` on the server is
  upgraded. (Andrew Bennetts, #109143)
"""

It's long, but it does render to HTML pretty well.

In fact, it feels like a very long way to say "finally it Just Works the way you'd intuitively expect it to". I guess it's not so bad to make this improvement extra visible...