check-code: move fixing up regexp into main procedure
This patch makes an extra check pattern to be prepared by
"_preparepats()" as similarly as existing patterns, if it is added to
"checks" array before invocation of "main()" in check-code.py.
This is a part of preparation for adding check-code.py extra checks by
another python script in subsequent patch.
This is also useful for SkeletonExtensionPlan.
https://www.mercurial-scm.org/wiki/SkeletonExtensionPlan
#require docutils gettext
$ $TESTDIR/check-gendoc ro
checking for parse errors