resolve: use "other" changeset from merge state (issue4163) We can use the "other" data from the recorded merge state instead of inferring what the other could be from working copy parent. This will allow resolve to fulfil its duty even when the second parent have been dropped. Most direct benefit is fixing a regression in backout.

merge: add "other" file node in the merge state file This data is mostly redundant with the "other" changeset node (+ other changeset file path). However, more data never hurt. The old format do not store it so this require some dancing to add and remove it on demand.