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
-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----