rebase: when using --keep, don't care about pruned commits or divergence
`hg rebase --keep` creates duplicate commits (not successors), so I
was surprised that it still skips pruned commits and errors out if it
"would cause divergence" (it wouldn't). I guess this was just an
oversight. We didn't have any tests for it, so I also included that.
Differential Revision: https://phab.mercurial-scm.org/D10269
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