rebase: avoid converting from nodes to revnums twice
In the case where the node has successors, but none of them is an
ancestor of the destination, we would iterate over the successor nodes
twice, check if they're in the repo and convert them to revnums. I
doubt it's a measureable cost, but it gets simpler this way too.
Differential Revision: https://phab.mercurial-scm.org/D3941
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