evolvecmd: clear mergestate ourselves in _resolvephasedivergent()

`repo.commitctx()` does not clear the mergestate. We need to clear the
mergestate ourselves here.

The issue poped up when we started having mergestate with extras and no
conflicted files. Storage of these extras was recently added to mergestate.
8 jobs for topic/default/clear-ms in 6 minutes and 12 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Test
passed checks-py2 #105539

00:01:47

passed checks-py3 #105540

00:01:39

failed doc #105545

00:00:18

manual pytype #105538
allowed to fail manual
failed tests-py2-cext #105541

00:04:49

failed tests-py2-pure #105542

00:06:00

failed tests-py3-cext #105543

00:06:05

failed tests-py3-pure #105544

00:05:21

 
Name Stage Failure
failed
tests-py2-pure Test
Skipped test-check-flake8.t: blacklisted
Skipped test-check-pyflakes.t: blacklisted
Skipped test-check-setup-manifest.t: blacklisted
Failed test-evolve-interrupted.t: output changed
# Ran 193 tests, 6 skipped, 1 failed.
python hash seed: 3280050837
Running after_script
Uploading artifacts for failed job
ERROR: Job failed: exit code 1
failed
tests-py3-pure Test
Skipped test-check-flake8.t: blacklisted
Skipped test-check-pyflakes.t: blacklisted
Skipped test-check-setup-manifest.t: blacklisted
Failed test-evolve-interrupted.t: output changed
# Ran 193 tests, 6 skipped, 1 failed.
python hash seed: 2045346256
Running after_script
Uploading artifacts for failed job
ERROR: Job failed: exit code 1
failed
tests-py3-cext Test
Skipped test-check-flake8.t: blacklisted
Skipped test-check-pyflakes.t: blacklisted
Skipped test-check-setup-manifest.t: blacklisted
Failed test-evolve-interrupted.t: output changed
# Ran 193 tests, 6 skipped, 1 failed.
python hash seed: 1597233123
Running after_script
Uploading artifacts for failed job
ERROR: Job failed: exit code 1
failed
tests-py2-cext Test
Skipped test-check-compat-strings.t: blacklisted
Skipped test-check-debian.t: blacklisted
Skipped test-check-flake8.t: blacklisted
Skipped test-check-pyflakes.t: blacklisted
Skipped test-check-setup-manifest.t: blacklisted
Failed test-evolve-interrupted.t: output changed
# Ran 193 tests, 6 skipped, 1 failed.
python hash seed: 3910235496
ERROR: Job failed: exit code 1
failed
doc Test
    raise error.Abort(msg, hint=hint)
Abort: no such help topic: evolve
The full traceback has been saved in /tmp/sphinx-err-5DtA4K.log, if you want to report the issue to the developers.
Please also report this if it was a user error, so that a better error message can be provided next time.
A bug report can be filed in the tracker at <https://github.com/sphinx-doc/sphinx/issues>. Thanks!
make: *** [makefile:5: all] Error 2
Running after_script
Uploading artifacts for failed job
ERROR: Job failed: exit code 1