evolve: clean merge state when content divergence resolution has no changes

This patch backs out b723805d7fe1, which only hid the problem.

Usually mergestate is cleared (or should be cleared) when creating a new
changeset, but sometimes there are situations when there's nothing to commit.

When solving content divergence resulted in no changes and no new changesets
were created, mergestate was left untouched. That's a problem because sometimes
mergestate contains files with conflicts that were already resolved by user,
and such mergestate only applies to the current wdir parent. Since evolve has
already dealt with the current wdir parent successfully when it reached this
code, this is a good place to clean the mergestate.

--HG--
branch : stable
7 jobs for topic/stable/fix-tip in 5 minutes and 1 second (queued for 1 second)
Status Job ID Name Coverage
  Test
passed doc #116509

00:00:40

passed flake8 #116503

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manual pytype #116504
allowed to fail manual
passed tests-py2-cext #116505

00:03:57

passed tests-py2-pure #116506

00:03:40

passed tests-py3-cext #116507

00:05:00

passed tests-py3-pure #116508

00:04:24