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]
> fastannotate=
> EOF
$ hg init repo
$ cd repo
changes to whitespaces
$ cat >> a << EOF
> 1
>
>
> 2
> EOF
$ hg commit -qAm '1'
$ cat > a << EOF
> 1
>
> 2
>
>
> 3
> EOF
$ hg commit -m 2
$ hg fastannotate -wB a
0: 1
0:
1: 2
0:
1:
1: 3