simplify filterfiles when filtering based on a directory since an unkown files cannot be an exact match, we bisect for a <path>/ instead of <path> and we get only the files below the directory.

filterfiles: Search as long as the target is a prefix of current. filterfiles was failing to find files for directory arguments if another file existed that started with the directory name and sorted earlier. For example, a manifest of ('foo.h', 'foo/foo') would cause filterfiles('foo') to return nothing. This resolves issue #294.