Deleting before fold no longer unfolds after undo

Use same row delta computation for updating start and
end rows in folds.

spec/app/display-buffer-spec.coffee

@@ -495,6 +495,20 @@ describe "DisplayBuffer", ->
           expect(displayBuffer.lineForRow(8).text).toMatch /^9-+/
           expect(displayBuffer.lineForRow(10).fold).toBeDefined()
 
+  describe "when the line being deleted preceeds a fold", ->
+    describe "when the command is undone", ->
+      it "restores the line and preserves the fold", ->
+        editSession.setCursorBufferPosition([4])
+        editSession.foldCurrentRow()
+        expect(editSession.isFoldedAtScreenRow(4)).toBeTruthy()
+        editSession.setCursorBufferPosition([3])
+        editSession.deleteLine()
+        expect(editSession.isFoldedAtScreenRow(3)).toBeTruthy()
+        expect(buffer.lineForRow(3)).toBe '    while(items.length > 0) {'
+        editSession.undo()
+        expect(editSession.isFoldedAtScreenRow(4)).toBeTruthy()
+        expect(buffer.lineForRow(3)).toBe '    var pivot = items.shift(), current, left = [], right = [];'
+
   describe ".clipScreenPosition(screenPosition, wrapBeyondNewlines: false, wrapAtSoftNewlines: false, skipAtomicTokens: false)", ->
     beforeEach ->
       displayBuffer.setSoftWrapColumn(50)

src/app/fold.coffee

@@ -36,8 +36,8 @@ class Fold
       @displayBuffer.unregisterFold(@startRow, this)
       return
 
-    @updateStartRow(event)
-    @updateEndRow(event)
+    @startRow += @getRowDelta(event, @startRow)
+    @endRow += @getRowDelta(event, @endRow)
 
     if @startRow != oldStartRow
       @displayBuffer.unregisterFold(oldStartRow, this)
@@ -49,26 +49,12 @@ class Fold
   isContainedByFold: (fold) ->
     @isContainedByRange(fold.getBufferRange())
 
-  updateStartRow: (event) ->
+  getRowDelta: (event, row) ->
     { newRange, oldRange } = event
 
-    if oldRange.end.row < @startRow
-      delta = newRange.end.row - oldRange.end.row
-    else if newRange.end.row < @startRow
-      delta = newRange.end.row - @startRow
+    if oldRange.end.row <= row
+      newRange.end.row - oldRange.end.row
+    else if newRange.end.row < row
+      newRange.end.row - row
     else
-      delta = 0
-
-    @startRow += delta
-
-  updateEndRow: (event) ->
-    { newRange, oldRange } = event
-
-    if oldRange.end.row <= @endRow
-      delta = newRange.end.row - oldRange.end.row
-    else if newRange.end.row <= @endRow
-      delta = newRange.end.row - @endRow
-    else
-      delta = 0
-
-    @endRow += delta
+      0