obsolete: detect divergent changesets
Divergent changeset are final successors (non obsolete) of a changeset who
compete with another set of final successors for this same changeset.
For example if you have two obsolescence markers A -> B and A -> C, B and C are
both "divergent" because they compete to be the one true successors of A.
Public revision can't be divergent.
This function is used and tested in the next changeset.
(binary:application/octet-stream)