py3: change default priority and length used for sorting hooks to be compatible with python 3
The call to sorted(hooks.values()) can on line 213 of hooks.py can raise when using
python 3. For instance, when hooks.values is `[(0, 2, b'post-commit.check-status', b''),
(None, None, b'changegroup.app-hooks', <object object at 0x7f5279885590>)]`, the error is
TypeError: '<' not supported between instances of 'NoneType' and 'int'
This fix keeps the same order that was used in python 2 without relying on comparison with
Differential Revision: https://phab.mercurial-scm.org/D8527