rebase: extract function for rebasing a single node

Authored by martinvonz.

Description

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

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