Improve source positions for lists

[jgm]

 - a
 - b

yields

doc
  list (1:2:2-4:1:11) list_style="-" tight="true"
    list_item (1:2:2-2:2:7)
      para (1:4:4-2:1:6)
        str (1:4:4-1:4:4) text="a"
    list_item (2:2:7-4:1:11)
      para (2:4:9-2:5:10)
        str (2:4:9-2:4:9) text="b"
references = {
}
footnotes = {
}

Ideally the first list item would end at 2:1 or 1:4 rather than 2:2.