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
#require docutils gettext
$ $TESTDIR/check-gendoc ru
checking for parse errors