metaedit: use pre-checking before rewriting csets

Changes in test file demonstrate the changed behaviour.
6 jobs for topic/default/precheck-metaedit in 4 minutes and 52 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Test
passed #53163
doc

00:00:25

passed #53158
flake8

00:00:23

passed #53159
tests-py2-cext

00:03:23

passed #53160
tests-py2-pure

00:04:00

passed #53161
tests-py3-cext

00:02:53

passed #53162
tests-py3-pure

00:03:29