Fix DiffTreeWalk to report parent trees if so instructed and there's a single node path filter.

Also reverts AddOp's conditional added. The actual solution is to
fix DiffTreeWalk to report parent trees as instructed by WorkTree.getUnstaged().
This was not so in the case there was a single path filter, so DiffTreeWalk
entered an optimized code path that didn't took into account if the boolean
reportTrees flag was set.
Merge pull request #9 from groldan/volaya-add_path

Fix DiffTreeWalk to report parent trees if so instructed and there's a single node path filter