check-code: factor out boot procedure into main

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

@@ -622,7 +622,7 b' def checkfile(f, logfunc=_defaultlogger.'
 
     return result
 
-if __name__ == "__main__":
+def main():
     parser = optparse.OptionParser("%prog [options] [files]")
     parser.add_option("-w", "--warnings", action="store_true",
                       help="include warning-level checks")
@@ -650,4 +650,7 b' if __name__ == "__main__":'
                          blame=options.blame, debug=options.debug,
                          lineno=options.lineno):
             ret = 1
-    sys.exit(ret)
+    return ret
+
+if __name__ == "__main__":
+    sys.exit(main())