tests/test-xdg.t

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

#if no-windows no-osx

  $ mkdir -p xdgconf/hg
  $ echo '[ui]' > xdgconf/hg/hgrc
  $ echo 'username = foobar' >> xdgconf/hg/hgrc
  $ XDG_CONFIG_HOME="`pwd`/xdgconf" ; export XDG_CONFIG_HOME
  $ unset HGRCPATH
  $ hg config ui.username 2>/dev/null
  foobar

#endif