Rename UndoManager.perform -> pushOperation

I'm about to expand the types of operations so that some don't always have a `do` method. Therefore `perform` is a misnomer.

src/app/buffer.coffee

@@ -133,7 +133,7 @@ class Buffer
     oldRange = Range.fromObject(oldRange)
     operation = new BufferChangeOperation({buffer: this, oldRange, newText})
     if @undoManager
-      @undoManager.perform(operation)
+      @undoManager.pushOperation(operation)
     else
       operation.do()
 

src/app/undo-manager.coffee

@@ -13,7 +13,7 @@ class UndoManager
     @undoHistory = []
     @redoHistory = []
 
-  perform: (operation) ->
+  pushOperation: (operation) ->
     if @currentBatch
       @currentBatch.push(operation)
     else