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.
Test update logic when there are renames
Update with local changes across a file rename
$ hg init
$ echo a > a
$ hg add a
$ hg ci -m a
$ hg mv a b
$ hg ci -m rename
$ echo b > b
$ hg ci -m change
$ hg up -q 0
$ echo c > a
$ hg up
merging a and b to b
warning: conflicts during merge.
merging b incomplete! (edit conflicts, then use 'hg resolve --mark')
0 files updated, 0 files merged, 0 files removed, 1 files unresolved
use 'hg resolve' to retry unresolved file merges
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