BzrDir.find_branches() goes into an infinite loop on MemoryTransport

If I run BzrDir.find_branches() on a MemoryTransport that contains directories, I see an infinite loop.

A simple test program that triggers the bug is:

    from bzrlib.bzrdir import BzrDir
    from bzrlib.transport import get_transport
    from bzrlib.transport.memory import MemoryServer

    server = MemoryServer()
    server.setUp()
    transport = get_transport(server.get_url())
    BzrDir.create_branch_and_repo(transport.abspath('.'))
    for branch in BzrDir.find_branches(transport):
        print branch.base

Adding a few debug prints shows that MemoryTransport.list_dir() is always returning the directories at the base of the tree, irrespective of the transport's base URL.

So in the above example, it effectively does:
 1. checks to see if memory+NNN:/// is a branch (it is).
 2. lists memory+NNN:///, getting ['.bzr'] as a result.
 3. checks to see if memory+NNN:///.bzr/ is a branch (it is not)
 4. lists memory+NNN:///.bzr/, getting ['.bzr'] as a result.
 5. checks to see if memory+NNN:///.bzr/.bzr/ is a branch (it isn't -- the path does not exist)
 6. lists memory+NNN://.bzr/.bzr/, getting ['.bzr'] as a result, despite the fact that the directory doesn't exist
 ...

This makes it impossible to use TestCaseWithMemoryTransport for tests that involve BzrDir.find_branches() or BzrDir.find_bzrdirs().