debugcommands: support for sending "batch" requests Let's teach `hg debugwireproto` to send "batch" requests. The easiest way to implement this was as a pair of instructions to begin and end a batched operation. Otherwise, we would have to reinvent the parsing wheel or factor out the parsing code. To prove it works, we add a batched request to test-ssh-proto.t. Differential Revision: https://phab.mercurial-scm.org/D2408