diff: migrate to modern pager API

hgext/pager.py

@@ -110,4 +110,4 b' def uisetup(ui):'
         extensions.wrapfunction(dispatch, '_runcommand', pagecmd)
     extensions.afterloaded('color', afterloaded)
 
-attended = ['diff', 'export', 'glog', 'log', 'qdiff']
+attended = ['export', 'glog', 'log', 'qdiff']

mercurial/commands.py

@@ -1940,6 +1940,7 b' def diff(ui, repo, *pats, **opts):'
 
     diffopts = patch.diffallopts(ui, opts)
     m = scmutil.match(repo[node2], pats, opts)
+    ui.pager('diff')
     cmdutil.diffordiffstat(ui, repo, diffopts, node1, node2, m, stat=stat,
                            listsubrepos=opts.get('subrepos'),
                            root=opts.get('root'))