evolve: add pre-check logic for content-divergence in rewriteutil.precheck()
For now, pre-check will abort if rewriting a rev create divergence (and config experimental.evolution.allowdivergence is not set to True). But this behaviour can be improved where instead of abort maybe we can confirm the user to either proceed with divergence or some other options depends on what command user is running. Changes in test file are used to overrides the pre-check for testing purpose (using experimental.evolution.allowdivergence=yes).
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