changectx: add a "maybe filtered" filtered attribute
There are changeset that we know not to be filtered (eg: `null`). In this case,
we could access some information without triggering changelog filtering. We add
an attribute to changectx that track this property.
Differential Revision: https://phab.mercurial-scm.org/D7483
cat >> $HGRCPATH <<EOF
[extensions]
narrow=
[ui]
ssh=python "$RUNTESTDIR/dummyssh"
[experimental]
changegroup3 = True
EOF