fix bugs in new function for moving cursor upwards, taking care to calculate column different if on first line (where there is no preceding newline) vs any line other than the first

test/normal-move.sml

@@ -303,22 +303,6 @@ struct
            in
              Expect.isTrue isSkipped
            end)
-    , test "moves to 0 of buffer when on first line" (fn _ =>
-        let
-          (* arrange *)
-          val str = "hello \nworld \ntime to go\n"
-          val buffer = LineGap.fromString str
-          val app = TestUtils.init buffer
-          val app = AppWith.idx (app, 5)
-
-          (* act *)
-          val {cursorIdx, ...} = TestUtils.update (app, CHAR_EVENT #"k")
-
-          (* assert *)
-          val isAtStart = cursorIdx = 0
-        in
-          Expect.isTrue isAtStart
-        end)
     , test "leaves cursor at same idx when already at start of buffer" (fn _ =>
         let
           (* arrange *)