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
#require docutils gettext
$ $TESTDIR/check-gendoc fr
checking for parse errors