exchange: fix dead assignment

The assignment of the value from bundle2.processbundle() to 'r' is
unused. It is currently the same as its third argument (if given), and
since that argument may eventually go away (according to the method's
docstring), let's reassign the return value to 'op' instead to better
prepare for that.

mercurial/exchange.py

@@ -1419,7 +1419,7 b' def unbundle(repo, cg, heads, source, ur'
                 op = bundle2.bundleoperation(repo, lambda: tr,
                                              captureoutput=captureoutput)
                 try:
-                    r = bundle2.processbundle(repo, cg, op=op)
+                    op = bundle2.processbundle(repo, cg, op=op)
                 finally:
                     r = op.reply
                     if captureoutput and r is not None: