[Toybox] tar test failures

[enh]

i haven't had time to look properly (and probably won't for a couple of
weeks), but this change seems to have badly broken tar, causing lots of the
tar tests to crash:

commit 0d73d98537e4c058537ffa735b8592d1a89a1028
Author: Rob Landley <rob at landley.net>
Date:   Sat Sep 26 23:42:38 2020 -0500

    Teach dirtree that top level "" node is "." but not show in
dirtree_path().
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