rebase: extract function for rebasing a single node We currently have _performrebase() and _performrebasesubset(), but we don't have a method for rebasing a single node (that's inside a loop in _performrebasesubset()). I think it makes sense to have such a method, so that's what this patch does. I think it may simplify future patches I'm working on that have to do with transactions, but I think this patch makes sense on its own whether or not that future work happens. Differential Revision: https://phab.mercurial-scm.org/D2809