dirstate: use a new `drop_merge_data` in `setparent` What is happening in this `setparent` loop is that we remove all `merge` related information when the dirstate is moved out of a `merge` situation. So instead of shuffling state to get them where we want, we simply add a method on the DirstateItem to do drop the information we want dropped. Differential Revision: https://phab.mercurial-scm.org/D11506