drawdag: abide by new createmarkers() API The `obsolete.createmarkers()` API was changed in 6335c0de80fa (obsolete: allow multiple predecessors in createmarkers, 2018-09-22) to prefer its precursors input to be a tuple instead of a single precursor. Let's fix `drawdag.py` to comply. Differential Revision: https://phab.mercurial-scm.org/D8149

cat >> $HGRCPATH <<EOF
[extensions]
narrow=
[ui]
ssh=python "$RUNTESTDIR/dummyssh"
[experimental]
changegroup3 = True
EOF