revset: remove order information from tree (API)
Keeping `order` in tree makes AST operation harder. And there could be
invalid cases if trees could be generated and compounded freely, like:
SetA(order=define) & SetB(order=define)
^^^^^^ couldn't be satisfied
This patch changes the code to calculate order on the fly, during tree
traversal. Optimization of reordering `and` arguments is preserved by
introducing a new internal operation `flipand`.
.. api::
revset.stringset() now takes 'order' as the last argument.
Differential Revision: https://phab.mercurial-scm.org/D451