extensions: use [0:1] slice on config path instead of [0] This behaves the same in Python 2 and Python 3, even though the path is a bytes.

extensions: use inspect module instead of func_code.co_argcount Fixes the extsetup argspec check on Python 3.

treemanifest: add tests covering hg diff of partial trees Previously the hg files tests also covered the logic (i.e. treemanifest.matches) that governed how hg diff limited its diff. In a future patch we will be switching treemanifest.diff() to have a custom implementation, so let's go ahead and add equivalent test coverage for hg diff.