Join of second upgraded tree fails with "File id {TREE_ROOT} already exists in inventory"

This one only occurs (as far as I can tell) if the trees being joined
were at some point upgraded from pack-0.92 to 2a, either by `bzr
upgrade' or implicitly during the `bzr join'. On the attmped to join
the second tree, an error like the following occurs:

    bzr: ERROR: File id {TREE_ROOT} already exists in inventory as InventoryDirectory('TREE_ROOT', u'p1', parent_id='tree_root-20101224155900-o4tj2tdno7n14m4t-1', revision=None)

p1 here is the name of the first tree, which was successfully joined.
The failure occurs on `bzr join p2'.

Tested in Bzr 2.2.0 on Windows.

The attached script refers to this bug as the join-of-second-converted-tree bug.
The script also mentions the join-without-prior-commit bug (#694161)
because it is necessary to work around that one to demonstrate this one.