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

Thanks for the review. I have made the following changes
* Renamed overly verbose error class
* Made all text lines less than 80 chars long
* Added docstring to is_realpath_outside_tree()
* Removed use of os.mkdir where appropriate in tests

I have the following work left to do
* Add a couple of tests directly for is_realpath_outside_tree()

I have the following questions
* When you talk of adding tests directly for is_realpath_outside_tree() I take it that you mean adding more blackbox tests to test_add.py. Is this correct?
* "You should have two blank spaces before the def" Why? Surely there should be no spaces before the def - in the same way as the function that follows it (def tree_files) has no preceding spaces
* Would is_realpath_outside_tree() be better off in osutils.py?