save-cmd: don't fail an assertion when doing a backup from the root level.

save-cmd: don't fail an assertion when doing a backup from the root level.

This wasn't caught by unit tests because "virtual" nodes added by
index.py:_golevel() weren't being marked as IX_EXISTS, which in the unit
tests included the root, so save-cmd was never actually trying to back up
that node.

That made the base directories incorrectly marked as status=D (deleted) if
you printed out the index during the tests.  So add a test for that to make
it fail if "/" is deleted (which obviously makes no sense), then add another
test for saving from the root level, then fix both bugs.