evolve: remove unused struct import

compat: ask pytype to ignore some warnings It's expected to have a bunch of import and attribute errors in this module because it's for compatibility with different versions of hg and Python. Although it doesn't explain why a function wouldn't have __code__ attribute? So far I think it's a quirk of pytype.

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()`