hg2git: in _init_dirs, store keys without leading '/' (issue103)

Previously, whenever a tree that wasn't the root ('') was stored, we'd prepend
a '/' to it. Then, when we'd try retrieving the entry, we'd do so without the
leading '/'. This caused data loss because existing tree entries were dropped
on the floor. Fix that by only adding '/' if we're adding to a non-empty
initial path.

This wasn't detected in tests because most of them deal only with files in the
root and not ones in subdirectories.
tests: add some extra filters for new output

Tests pass on both my Mac and Linux machine, so I feel moderately confident
that this is portable.