py3: use stringutil.pprint() instead of '%r'

This makes test-pager-legacy.t pass on Python 3.

Differential Revision: https://phab.mercurial-scm.org/D5037

contrib/python3-whitelist

@@ -392,6 +392,7 b' test-obsolete-distributed.t'
 test-obsolete-divergent.t
 test-obsolete-tag-cache.t
 test-obsolete.t
+test-pager-legacy.t
 test-pager.t
 test-parents.t
 test-parseindex2.py

mercurial/ui.py

@@ -1076,7 +1076,8 b' class ui(object):'
             if name not in encoding.environ:
                 pagerenv[name] = value
 
-        self.debug('starting pager for command %r\n' % command)
+        self.debug('starting pager for command %s\n' %
+                   stringutil.pprint(command))
         self.flush()
 
         wasformatted = self.formatted()