revset: add helper to build balanced addsets from chained 'or' operations This function will be used by revset.orset() and scmutil.revrange() to reduce the stack depth from O(n) to O(log(n)). We've bikeshed the interface of this function, but we couldn't come to an agreement. So we decided to attempt to make it move forward. marmoute: - new factory function isn't necessary for balanced addsets - addset.__init__ can just recurse, should handle "len(subsets) == 2+" yuja: - want to write all "len(subsets) == 0, 1, 2, 3+" cases in the same function - no recursion in __init__ for cosmetic reason: can't return, can't call __init__ directly I've changed it to a private function so that nobody would be tempted to utilize it.