[PATCH 2 of 2 rfc] subrepos: make subrepositories sticky

[timeless]

>                    _('warning: subrepository %s left in revision %s\n') %

please use <at> <revision> instead of <in> <revision>

>+                self._ui.warn(_('Some hint???\n'))

linguists can't work from this, please use XXX and a long string that
someone can try to understand. if i see XXX and something long, i'll
try to work on it. you can also cc me on messages where you need help
(no guarantees, but it will call it out to me...)