discovery: move code to create outgoing from roots and heads changegroup.changegroupsubset() contained somewhat low-level code for constructing an "outgoing" instance from a list of roots and heads nodes. It feels like discovery.py is a more appropriate location for this code. This code can definitely be optimized, as outgoing.missing will recompute the set of changesets we've already discovered from cl.between(). But code shouldn't be refactored during a move, so I've simply inserted a TODO calling attention to that.

#require docutils gettext

  $ $TESTDIR/check-gendoc de
  checking for parse errors
  Die Dateien werden dem Projektarchiv beim n\xc3\xa4chsten \xc3\x9cbernehmen (commit) hinzugef\xc3\xbcgt. Um dies vorher r\xc3\xbcckg\xc3\xa4ngig zu machen, siehe:hg:`forget`. (esc)
  warning: please have a space before :hg: