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
{
// Enforcing
"eqeqeq" : true, // true: Require triple equals (===) for comparison
"forin" : true, // true: Require filtering for..in loops with obj.hasOwnProperty()
"freeze" : true, // true: prohibits overwriting prototypes of native objects such as Array, Date etc.
"nonbsp" : true, // true: Prohibit "non-breaking whitespace" characters.
"undef" : true, // true: Require all non-global variables to be declared (prevents global leaks)
// Environments
"browser" : true // Web Browser (window, document, etc)
}