Code review comment for lp://staging/~declanmg/bzr/264275-fix

More work committed and pushed to branch. Here are some questions

* I have added 2 tests for is_realpath_outside_tree(). To exercise the method I had to 'import bzrlib.builtins' at the top of the tests/blackbox/test_add.py
  - Please advise if this is ok. It may violate the spirit of a blackbox test.

* Alternatively, I can move the method to osutils.py. I was thinking I could reverse the method's logic and rename it to is_realfilepath_in_realfolderpath() to make it more generic (as it doesn't need to be tree specific).