Work around "bzr ls --non-recursive PATH : no list" bug in old bzrlib.

Work around "bzr ls --non-recursive PATH : no list" bug in old bzrlib.

See: https://bugs.launchpad.net/bzr/+bug/158690

Bug affected versions:
  0.90.0 (reported)
  1.3.1  (my test suite hit it)
Doesn't affect versions:
  2.0+   (non_recursive -> recursive)
But I haven't isolated the source more specifically.

Working around it for everything < 2.0 should be safe, but the cutoff
could be fine-tuned if someone wants to dig through the bzr.dev
history...