changegroup: use positive logic for treemanifest changegroup3 logic
We are about to add more cases, in that context, it is simpler to avoid double
negative.
En addition, We document the situation for the next soul.
Differential Revision: https://phab.mercurial-scm.org/D6938
#require docutils gettext
$ $TESTDIR/check-gendoc el
checking for parse errors