dirstate: use 'm' state in otherparent to reduce ambiguity In rebase-like operations where we abandon the second parent, we can correctly fix up the state in setparents.

dirstate: properly clean-up some more merge state on setparents

phases: move root phase assignment to it's own function This moves the initial root phase assignment to it's own function. Future patches which make phase calculations lazy will use this function to pre-fill certain phases which can be deduced from the roots.