Jan Vrany <jan.vrany@fit.cvut.cz> [Thu, 29 Mar 2018 22:22:31 +0100] rev 816
Fixed `HGChangeset >> #successors` for Mercurial < 4.1
...which does not support `successors()` in revset. In this case,
return an empty successors and don't show "reconcile" button in
commit dialog.
There well may be other way to get successors on Mercurial 4.0, just
I'm not aware of it. Once found, this limitation will be lifted.