bundle2: only use callable return as reply handler

When a bundle2 parts generator returns a non callable value, it should not be
used as a reply handler. The changegroup part generator is already having this
case of behavior when there is no changegroup to push. This changeset prevent a
crash for user of the experimentable bundle2 feature.

mercurial/exchange.py

@@ -242,7 +242,8 b' def _pushbundle2(pushop):'
     replyhandlers = []
     for partgen in bundle2partsgenerators:
         ret = partgen(pushop, bundler)
-        replyhandlers.append(ret)
+        if callable(ret):
+            replyhandlers.append(ret)
     # do not push if nothing to push
     if bundler.nbparts <= 1:
         return

tests/test-bundle2.t

@@ -957,7 +957,6 b' Setting up'
   >     if reason == 'race':
   >         # 20 Bytes of crap
   >         bundler.newpart('b2x:check:heads', data='01234567890123456789')
-  >     return lambda op: None
   > 
   > @bundle2.parthandler("test:abort")
   > def handleabort(op, part):