rewriteutil: check for divergence This code is adapted from the code in the evolve extension. It seems to be equivalent as far as the evolve extension's test suite can tell (the only impact when making their `precheck()` delegate to our version is that error messages are less detailed). I had to change the error message to work with "change branch of" being inserted as the action. Differential Revision: https://phab.mercurial-scm.org/D10518

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