check-code: make it possible to ignore the PWD check in some situation

The check-code script does not motivate the banning of $PWD so I am not sure
what it is about. And I will needs to use the env variable in the next tests. So
I am adding a way to make an exception.

Differential Revision: https://phab.mercurial-scm.org/D11450

contrib/check-code.py

@@ -195,7 +195,7 b' testpats = ['
     [
         (r'^function', "don't use 'function', use old style"),
         (r'^diff.*-\w*N', "don't use 'diff -N'"),
-        (r'\$PWD|\${PWD}', "don't use $PWD, use `pwd`"),
+        (r'\$PWD|\${PWD}', "don't use $PWD, use `pwd`", "no-pwd-check"),
         (r'^([^"\'\n]|("[^"\n]*")|(\'[^\'\n]*\'))*\^', "^ must be quoted"),
         (r'kill (`|\$\()', "don't use kill, use killdaemons.py"),
     ],