Change the way methods that take a path argument behave.

Change the way methods that take a path argument behave.

For a number of user facing methods, make it possible to pass in paths in as
either a unicode string, or bytes. These include:

* Repo.__init__
* Repo.init
* Repo.init_bare
* index.build_index_from_tree
* index.get_unstaged_changes

The repo.Repo.path attribute will remain unchanged. The Repo._controldir
attribute and a new Repo._path_bytes attribute are ensured to be bytes.

For a number of internal methods, it now requires a bytes path rather than
a unicode string. These include:

* objects.hex_to_filename
* objects.filename_to_hex
* _GitFile.__init__
* ShellHook.__init__ (and subclassed of ShellHook)
* DiskObjectStore.__init__
* DiskRefsContainer.__init__
* Repo._put_named_file
* Repo.get_named_file
* pack.write_pack
* Pack.__init__
* etc...