match: add match.ispartial()
match.ispartial() will return the opposite of match.always() in core, but this
function will be extensible by extensions to produce another result even
if match.always() will be untouched.
This will be useful for narrowhg, where ispartial() will return False even if
the match won't always match. This would happen in the case where the only
time the match function is False is when the path is outside of the narrow
spec.