When snippet expansion is undone, the snippet is destroyed

This is still in progress. You can't *redo* snippet expansion and restore tab stops. Also, this commit performs all changes associated with snippet expansion in a transaction.

src/app/buffer.coffee

@@ -142,9 +142,9 @@ class Buffer
     @lines[startRow..endRow] = newLines
     @modified = true
 
-  pushOperation: (operation) ->
+  pushOperation: (operation, editSession) ->
     if @undoManager
-      @undoManager.pushOperation(operation)
+      @undoManager.pushOperation(operation, editSession)
     else
       operation.do()
 

src/app/edit-session.coffee

@@ -238,16 +238,19 @@ class EditSession
   transact: (fn) ->
     @buffer.transact =>
       oldSelectedRanges = @getSelectedBufferRanges()
-      @buffer.pushOperation
+      @pushOperation
         undo: (editSession) ->
           editSession?.setSelectedBufferRanges(oldSelectedRanges)
 
       fn()
       newSelectedRanges = @getSelectedBufferRanges()
-      @buffer.pushOperation
+      @pushOperation
         redo: (editSession) ->
           editSession?.setSelectedBufferRanges(newSelectedRanges)
 
+  pushOperation: (operation) ->
+    @buffer.pushOperation(operation, this)
+
   getAnchors: ->
     new Array(@anchors...)
 

src/app/undo-manager.coffee

@@ -12,13 +12,13 @@ class UndoManager
     @undoHistory = []
     @redoHistory = []
 
-  pushOperation: (operation) ->
+  pushOperation: (operation, editSession) ->
     if @currentTransaction
       @currentTransaction.push(operation)
     else
       @undoHistory.push([operation])
     @redoHistory = []
-    operation.do?()
+    operation.do?(editSession)
 
   transact: (fn) ->
     if @currentTransaction