check-code: forbid mutable value for default argument

default value are common to all call. Using mutable value is a classical source
of bug in Python. We forbid it.

The regexp (Courtesy of Matt Mackall) is only catching such value on the first
line of a definition, but that will be good enough for now.

contrib/check-code.py

@@ -291,6 +291,8 b' pypats = ['
     (r'os\.path\.join\(.*, *(""|\'\')\)',
      "use pathutil.normasprefix(path) instead of os.path.join(path, '')"),
     (r'\s0[0-7]+\b', 'legacy octal syntax; use "0o" prefix instead of "0"'),
+    # XXX only catch mutable arguments on the first line of the definition
+    (r'def.*[( ]\w+=\{\}', "don't use mutable default arguments"),
   ],
   # warnings
   [