debugcommands: support connecting to HTTP peers
Now that we have the plumbing for logging socket activity, let's hook
it up to `hg debugwireproto` so we can collect low-level activity on
sockets.
The new code is a bit incomplete. But it is better than nothing:
`hg debugwireproto` is still heavily evolving.
The added test demonstrates some interesting behavior. For example,
we're calling socket.makefile() and doing I/O on that. TIL. We're also
sending an "Accept-Encoding: identity" request header.
Differential Revision: https://phab.mercurial-scm.org/D2726
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