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()`

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