@tomchristie It seems part of functioncommon_path was extracted from Python's standard librarycommonpath (https://hg.python.org/cpython/file/3.5/Lib/posixpath.py#l469)
If that's the case, perhaps we should add a reference.

It's no longer the same implementation (since the stdlib one is useless), but I'd be happy to see a comment outlining what the function does.