destutil: if wdp is obsolete, update to the closest non-obsolete ancestor
As the original comments suggest, using prune as a model here was an existing
idea, and now this patch implements it.
I think it would be even better to do what solveobswdp() from evolve does (in
short, it allows to update to a successor of the closest ancestor even if that
ancestor is obsolete), but that is outside of this series' scope.
Differential Revision: https://phab.mercurial-scm.org/D12099
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