debugcommands: support for triggering push protocol The mechanism for pushing to a remote is a bit more complicated than other commands. On SSH, we wait for a positive reply from the server before we start sending the bundle payload. This commit adds a mechanism to the "command" action in `hg debugwireproto` to trigger the "push protocol" and to specify a file whose contents should be submitted as the command payload. With this new feature, we implement a handful of tests for the "unbundle" command. We try to cover various server failures and hook/output scenarios so protocol behavior is as comprehensively tested as possible. Even with so much test output, we only cover bundle1 with Python hooks. There's still a lot of test coverage that needs to be implemented. But this is certainly a good start. Because there are so many new tests, we split these tests into their own test file. In order to make output deterministic, we need to disable the doublepipe primitive. We add an option to `hg debugwireproto` to do that. Because something in the bowels of the peer does a read of stderr, we still capture read I/O from stderr. So there is test coverage of what the server emits. The tests around I/O capture some wonkiness. For example, interleaved ui.write() and ui.write_err() calls are emitted in order. However, (presumably due to buffering), print() to sys.stdout and sys.stderr aren't in order. We currently only test bundle1 because bundle2 is substantially harder to test because it is more complicated (the server responds with a stream containing a bundle2 instead of a frame). Differential Revision: https://phab.mercurial-scm.org/D2471