debugcommands: introduce debugpeer command
`hg debugpeer <path>` will establish a connection to a peer repository
and print information about it.
If you add --debug, it will log low-level protocol request info. This
will be useful for upcoming tests around protocol handshaking.
Differential Revision: https://phab.mercurial-scm.org/D2025
#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