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

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 #626 from volaya/add_path

Ensure that trees are staged when adding to index using a path filter for a feature