rewriteutil: use precheck from core if recent enough

This commit makes the evolve extension's `rewriteutil.precheck()`
simply delegate to the same function from Mercurial core, if that
function is deemed recent enough. The way I made that check is by
looking for `rewriteutil.find_new_divergence_from()`. That means that
there's a range of commits (ba6881c6::8125bcd2^) where the user would
see less detailed error messages if they are running with an hg
version from that range (there are no released versions in that

As you can see in the tests, switching to core's version of the
function mostly affects error messages and exit codes.
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