httppeer: add TODO about delayed handling of ^C

When testing wire protocol v2 with large responses, I noticed that
^C doesn't result in immediate termination. I'm not sure what the
root cause is. Let's add a TODO so we don't forget about it.

Differential Revision: https://phab.mercurial-scm.org/D4438

mercurial/httppeer.py

@@ -728,6 +728,8 b' class httpv2executor(object):'
         if not self._responsef:
             return
 
+        # TODO ^C here may not result in immediate program termination.
+
         try:
             self._responsef.result()
         finally: