check-code: prohibit `if False` antipattern

Differential Revision: https://phab.mercurial-scm.org/D20

contrib/check-code.py

@@ -301,6 +301,7 b' pypats = ['
      "comparison with singleton, use 'is' or 'is not' instead"),
     (r'^\s*(while|if) [01]:',
      "use True/False for constant Boolean expression"),
+    (r'^\s*if False(:| +and)', 'Remove code instead of using `if False`'),
     (r'(?:(?<!def)\s+|\()hasattr\(',
      'hasattr(foo, bar) is broken on py2, use util.safehasattr(foo, bar) '
      'instead', r'#.*hasattr-py3-only'),