merge: infer the "other" changeset when falling back to v1 format When we have to fallback to the old version of the file, we infer the "other" from current working directory parent. The same way it is currently done in the resolve command. This is know to have shortcoming… but we cannot do better from the data contained in the old file format. This is actually the motivation to add this new file format.

merge: record the "other" node in merge state We need to record the merge we were merging with. This solve multiple bug with resolve when dropping the second parent after a merge. This happen a lot when doing special merge (overriding the ancestor). Backout, shelve, rebase, etc. can takes advantage of it. This changeset just add the information in the merge state. We'll use it in the resolve process in a later changeset.