Adjust replace mode backspace to work w/ new history API

As of atom/text-buffer#81, which will ship in Atom v1.0.3, the editor's history
API has changed in some subtle ways in order to preserve the proper nesting of
transactions. The `groupChangesSinceCheckpoint` method no longer works if called
from within a transaction with a checkpoint that was created *outside* the
transaction.

/cc @jacekkopecky, can you double check that this alternative approach works
correctly?

Author: maxbrunsfeld

lib/vim-state.coffee

@@ -430,7 +430,6 @@ class VimState
 
   activateReplaceMode: ->
     @activateInsertMode('replace')
-    @replaceModeCheckpoint = null
     @replaceModeCounter = 0
     @editorElement.classList.add('replace-mode')
     @subscriptions.add @replaceModeListener = @editor.onWillInsertText @replaceModeInsertHandler
@@ -439,20 +438,20 @@ class VimState
   replaceModeInsertHandler: (event) =>
     chars = event.text?.split('') or []
     selections = @editor.getSelections()
-    @replaceModeCheckpoint = @editor.createCheckpoint()
     for char in chars
       continue if char is '\n'
       for selection in selections
-        # Delete next character
         selection.delete() unless selection.cursor.isAtEndOfLine()
+    return
 
   replaceModeUndoHandler: (event) =>
-    @editor.groupChangesSinceCheckpoint(@replaceModeCheckpoint) if @replaceModeCheckpoint?
     @replaceModeCounter++
 
   replaceModeUndo: ->
     if @replaceModeCounter > 0
       @editor.undo()
+      @editor.undo()
+      @editor.moveLeft()
       @replaceModeCounter--
 
   setInsertionCheckpoint: ->

spec/operators-spec.coffee

@@ -1831,9 +1831,11 @@ describe "Operators", ->
       keydown 'backspace', raw: true
 
       expect(editor.getText()).toBe "12foo345\n67890"
+      expect(editor.getSelectedText()).toBe ""
 
       keydown 'backspace', raw: true
       expect(editor.getText()).toBe "12foo345\n67890"
+      expect(editor.getSelectedText()).toBe ""
 
     it "can be repeated", ->
       keydown "R", shift: true