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
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