graft: move commit info building This moves the logic for generating the commit metadata ahead of the merge operation. The only purposae of this patch is to make subsequent patches easier to read, and there should be no behavior changes.

merge with stable

merge: support calculating merge actions against non-working contexts This is not currently used. It is instead a pre-requisite to performing non-conflicting grafts in memory, which a subsequent patch will do.

merge: refactor action calculation into function This pulls the code used to calculate the changes that need to happen during merge.update() into a separate function. This is not useful on its own, but is instead preparatory to performing grafts in memory when there are no potential conflicts.