evolve: extract logic of pre-checking divergence risk to its own function Extracting the logic (which is used to check if rewriting an obsolete revision will create divergence) to a function. This function will be used in upcoming patches of pre-checking. This is how we check if rewriting an obsolete revision will create divergence. We need to check two cases: If there is any 1) non-obsolete successor. -> easily done by `obsutil.successorssets()` 2) precursor with non-obsolete successor. -> using `evolvecmd.divergentsets()`

test-drop.t
test-inhibit.t
test-simple4server.t
tests/test-simple4server-bundle2.t