Auto merge of #4230 - Mark-Simulacrum:double-print, r=alexcrichton
authorbors <bors@rust-lang.org>
Tue, 27 Jun 2017 00:42:29 +0000 (00:42 +0000)
committerbors <bors@rust-lang.org>
Tue, 27 Jun 2017 00:42:29 +0000 (00:42 +0000)
Prevent rustc stderr/stdout from being duplicated.

Please review carefully. I've not submitted patches to Cargo before, I think, so this may be flawed in some way I haven't detected yet. Tests are green locally, though.

Fixes #4223


Trivial merge