Deep sub-directory support for group/namespace

so group path could be set freely such as:
    a -> a/b/c,
    a/b/c -> a

useful to host some complicated repositories tree layout,
e.g.: Google Android repositories tree.

see https://github.com/mhfan/gitlabhq/commit/4b07511a4f27dedd6b2657b40e7d609bbbf1eb91