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.
$ hg init
$ echo 123 > a
$ hg add a
$ hg commit -m "first" a
$ mkdir sub
$ echo 321 > sub/b
$ hg add sub/b
$ hg commit -m "second" sub/b
$ cat sub/b
321
$ hg co 0
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
$ cat sub/b 2>/dev/null || echo "sub/b not present"
sub/b not present
$ test -d sub || echo "sub not present"
sub not present