bundle2: flush output in a part in all cases

We want to preserve output even when the unbundling fails (eg: hook output). So
we must make sure that everything we have is flushed into the reply bundle.

(This is related to issue4594)

mercurial/bundle2.py

@@ -357,10 +357,10 b' def _processpart(op, part):'
         finally:
             if output is not None:
                 output = op.ui.popbuffer()
-        if output:
-            outpart = op.reply.newpart('output', data=output,
-                                       mandatory=False)
-            outpart.addparam('in-reply-to', str(part.id), mandatory=False)
+            if output:
+                outpart = op.reply.newpart('output', data=output,
+                                           mandatory=False)
+                outpart.addparam('in-reply-to', str(part.id), mandatory=False)
     finally:
         # consume the part content to not corrupt the stream.
         part.seek(0, 2)