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.
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.