treemanifests: make _loadchildrensetlazy just call _loadlazy
This was suggested on the original review for _loadchildrensetlazy (D4370), it's
taken me a while to get to it though.
Differential Revision: https://phab.mercurial-scm.org/D4843
cat >> $HGRCPATH <<EOF
[extensions]
narrow=
[ui]
ssh=python "$TESTDIR/dummyssh"
[experimental]
changegroup3 = True
EOF