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

contrib/check-code.py

@@ -474,7 +474,6 b' def _preparepats():'
         filters = c[3]
         for i, flt in enumerate(filters):
             filters[i] = re.compile(flt[0]), flt[1]
-_preparepats()
 
 class norepeatlogger(object):
     def __init__(self):
@@ -644,6 +643,8 b' def main():'
     else:
         check = args
 
+    _preparepats()
+
     ret = 0
     for f in check:
         if not checkfile(f, maxerr=options.per_file, warnings=options.warnings,