pager: don't page stderr if it's being redirected (issue2541)
$ mkdir t $ cd t $ hg init $ echo a > a $ hg add a $ hg commit -m test $ rm .hg/requires $ hg tip abort: index 00changelog.i unknown format 2! [255] $ echo indoor-pool > .hg/requires $ hg tip abort: requirement 'indoor-pool' not supported! [255]