dirstate: use a new `drop_merge_data` in `setparent`...
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
