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.

spec/atom/line-folder-spec.coffee

@@ -320,6 +320,7 @@ describe "LineFolder", ->
     it "returns the nearest valid position based on the current screen lines", ->
       expect(folder.clipScreenPosition([-1, -1])).toEqual [0, 0]
       expect(folder.clipScreenPosition([0, -1])).toEqual [0, 0]
+      expect(folder.clipScreenPosition([-1, 5])).toEqual [0, 0]
       expect(folder.clipScreenPosition([1, 10000])).toEqual [1, 30]
       expect(folder.clipScreenPosition([2, 15])).toEqual [2, 15]
       expect(folder.clipScreenPosition([4, 32])).toEqual [4, 32]

spec/atom/line-wrapper-spec.coffee

@@ -244,6 +244,7 @@ describe "LineWrapper", ->
 
     it "disallows negative positions", ->
       expect(wrapper.clipScreenPosition([-1, -1])).toEqual [0, 0]
+      expect(wrapper.clipScreenPosition([-1, 10])).toEqual [0, 0]
       expect(wrapper.clipScreenPosition([0, -1])).toEqual [0, 0]
 
     it "disallows positions beyond the last row", ->

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