sshpeer: support not reading and forwarding stderr The "doublepipe" primitive as used by sshpeer will automatically read from stderr and forward output to the local ui. This poses problems for deterministic testing because reads may not be consistent. For example, the server may not be done sending all output to stderr and the client will perform different numbers of read operations or will read from stderr and stdout at different times. To make tests deterministic, we'll need to disable the "doublepipe" primitive and perform stderr I/O explicitly. We add an argument to the sshpeer constructor to disable the use of the doublepipe. Differential Revision: https://phab.mercurial-scm.org/D2467