rebase: avoid defining two lists with the same contents
In abort(), there's "dstates" and "rebased" that are identical, which
they seem to have been since
0806823370d8 (rebase: properly calculate
descendant set when aborting (
issue3332), 2012-03-22). Let's
de-duplicate. I don't know what "dstates" means, but "rebased" makes
sense (it's the list of rebased revisions), so let's pick that.
Differential Revision: https://phab.mercurial-scm.org/D2878
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