localrepo: reverse contexts in status
This is a slight tweak to how localrepo.status calculates what files have
changed. By forcing a changectx to be first operator and anything not a
changectx to be the second operator, we can later exploit this to allow
refactoring the status operation as a method of a context object.
Furthermore, this change will speed up 'hg diff --reverse' when used with the
working directory because the code will now hit a fast path without needing to
calculate an unneeded second manifest.