narrowbundle: use new context manager for silencing the ui
Same reasoning as the previous change. This affects a few tests
because of the hack from d7304434390f (changegroup: move message about
added changes to transaction summary, 2019-09-08).
Differential Revision: https://phab.mercurial-scm.org/D10886
[workspace]
members = ["hg-core", "hg-cpython", "rhg"]
exclude = ["chg", "hgcli"]