benchmarks: restore `output` variable lost in D10884

Noticed by pyflakes (which I didn't have installed for Python 3.9 when
I sent D10884).

Differential Revision: https://phab.mercurial-scm.org/D10894

contrib/benchmarks/__init__.py

@@ -76,8 +76,9 b' def runperfcommand(reponame, command, *a'
         ui, 'perfext', os.path.join(basedir, 'contrib', 'perf.py')
     )
     cmd = getattr(perfext, command)
-    with ui.silent():
-        cmd(ui, repo, *args, **kwargs)
+    ui.pushbuffer()
+    cmd(ui, repo, *args, **kwargs)
+    output = ui.popbuffer()
     match = outputre.search(output)
     if not match:
         raise ValueError("Invalid output {}".format(output))