Rely on clipScreenPosition in vertical movement methods

Before, we were manually clipping the position of the cursor in
vertical movement methods. Now we can just increment / decrement the
row and the position will be clipped when it is assigned.

Also, changed the definition of clip screen position to always 0 out
the column when the row is negative.

src/atom/cursor.coffee

@@ -50,11 +50,7 @@ class Cursor extends View
   moveUp: ->
     { row, column } = @getScreenPosition()
     column = @goalColumn if @goalColumn?
-    if row > 0
-      @setScreenPosition({row: row - 1, column: column})
-    else
-      @moveToLineStart()
-
+    @setScreenPosition({row: row - 1, column: column})
     @goalColumn = column
 
   moveDown: ->

src/atom/line-map.coffee

@@ -144,7 +144,12 @@ class LineMap
   clipScreenPosition: (screenPosition, eagerWrap) ->
     screenPosition = Point.fromObject(screenPosition)
 
-    screenPosition = new Point(Math.max(0, screenPosition.row), Math.max(0, screenPosition.column))
+    screenPosition.column = Math.max(0, screenPosition.column)
+
+    if screenPosition.row < 0
+      screenPosition.row = 0
+      screenPosition.column = 0
+
     maxRow = @lastScreenRow()
     if screenPosition.row > maxRow
       screenPosition.row = maxRow