debugbackupbundle: use new context manager for silencing the ui

A difference between setting `ui.quiet` and using `ui.silent()` is
that the latter also silences `ui.write()` calls. That's practically
always what one wants, including here, I think.

Differential Revision: https://phab.mercurial-scm.org/D10885

mercurial/debugcommands.py

@@ -3791,16 +3791,13 def debugbackupbundle(ui, repo, *pats, *
         if revs:
             revs = [other.lookup(rev) for rev in revs]
 
-        quiet = ui.quiet
+        with ui.silent():
         try:
-            ui.quiet = True
             other, chlist, cleanupfn = bundlerepo.getremotechanges(
                 ui, repo, other, revs, opts[b"bundle"], opts[b"force"]
             )
         except error.LookupError:
             continue
-        finally:
-            ui.quiet = quiet
 
         try:
             if not chlist: