cat: 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 = ['cat', 'diff', 'export', 'glog', 'log', 'qdiff']
+attended = ['diff', 'export', 'glog', 'log', 'qdiff']

mercurial/commands.py

@@ -1417,6 +1417,7 b' def cat(ui, repo, file1, *pats, **opts):'
     ctx = scmutil.revsingle(repo, opts.get('rev'))
     m = scmutil.match(ctx, (file1,) + pats, opts)
 
+    ui.pager('cat')
     return cmdutil.cat(ui, repo, ctx, m, '', **opts)
 
 @command('^clone',