ui: drop values returned by inspect.*frame*() to avoid cycles

"f = inspect.currentframe()" instantly creates a cycle because
"f.f_locals['f']" is "f" itself.

This patch explicitly sets those frame objects to None to avoid cycles.

mercurial/ui.py

@@ -1175,6 +1175,7 b' class ui(object):'
                            % ((msg,) + calframe[stacklevel][1:4]))
             self.log('develwarn', '%s at: %s:%s (%s)\n',
                      msg, *calframe[stacklevel][1:4])
+            curframe = calframe = None  # avoid cycles
 
     def deprecwarn(self, msg, version):
         """issue a deprecation warning