context: add __nonzero__ methods

mercurial/context.py

@@ -37,6 +37,9 b' class changectx(object):'
     def __eq__(self, other):
         return self._rev == other._rev
 
+    def __nonzero__(self):
+        return self._rev != -1
+
     def changeset(self):
         try:
             return self._changeset
@@ -129,6 +132,9 b' class filectx(object):'
         else:
             raise AttributeError, name
 
+    def __nonzero__(self):
+        return self._filerev != nullid
+
     def __str__(self):
         return "%s@%s" % (self.path(), short(self.node()))