wireprotoserver: add context manager mechanism for redirecting stdio Today, proto.redirect() sets up redirecting stdio and proto.restore() undoes that. The API is a bit wonky because restore() is only implemented on the HTTP protocol. Furthermore, not all calls to redirect() are obviously paired with calls to restore(). For example, the call to restore() for "unbundle" requests is handled by the response handler for the HTTP protocol. This commit introduces a new method on the protocol handler interface to maybe capture stdio. It emits a file object or None depending on whether stdio capture is used by the transport. To prove it works, the "pushkey" wire protocol command has been updated to use the new API. I'm not convinced this is the best mechanism to capture stdio. I may need to come up with something better once the new wire protocol emerges into existence. But it is strictly better than before because it removes variance in the wire protocol handler interface. It therefore gets us closer to a unified interface between the SSH and HTTP transports. Differential Revision: https://phab.mercurial-scm.org/D2081