Update linter to cope with CBMC subtree

Author: NathanJPhillips

scripts/cpplint.py

@@ -1152,7 +1152,7 @@ def RepositoryName(self):
       if (os.path.exists(os.path.join(root_dir, ".git")) or
           os.path.exists(os.path.join(root_dir, ".hg")) or
           os.path.exists(os.path.join(root_dir, ".svn"))):
-        prefix = os.path.commonprefix([root_dir, project_dir])
+        prefix = os.path.commonprefix([root_dir + "/cbmc", project_dir])
         return fullname[len(prefix) + 1:]
 
     # Don't know what to do; header guard warnings may be wrong...