evolve: refactor the code which swap two nodes (in divergence resolution) I see that we are using the code to swap the "divergent" and "other" node; and updating the evolvestate accordingly at many places in divergence resolution. This patch extract that code to a function. Although I don't have strong opinion on this patch. It's a volunteered patch to remove some redundancy.

syntax: re
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^html/
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~$
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^tests/.*\.err$
^tests/easy_run.sh$
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^docs/tutorials/.*\.rst$
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tests/\.testtimes

^docs/training/graphs/
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