practracker: An exception is "used" even when it is violated.

The purpose of tracking whether an exception is used is so that we
can tell whether it is overbroad.  This means that an _underbroad_
exception is still a used one.  Fixes bug 31338.

scripts/maint/practracker/problem.py

@@ -77,8 +77,10 @@ class ProblemVault(object):
         # (e.g. we went from 4k LoC to 3k LoC), but we do warn if the
         # situation worsened (e.g. we went from 60 includes to 80).
         status = problem.is_worse_than(self.exceptions[problem.key()])
-        if status == STATUS_OK:
-            self.used_exception_for[problem.key()] = problem
+
+        # Remember that we used this exception, so that we can later
+        # determine whether the exception was overbroad.
+        self.used_exception_for[problem.key()] = problem
 
         return status