debian: install hgk as part of mercurial-common (
issue4829)
Leaving the hgk binary in /usr/bin causes some lintian warnings, and
downstream packages poke it in /usr/share/mercurial, so we'll just
stash it in there. Rather than patch hgk.py as part of the Mercurial
install, just drop a config file in /etc/mercurial/hgrc.d that points
to the installed hgk.