Listen for changes to the CodeMirror Doc, instead of all changes to the editor

This prevents an issue where switching docs (i.e. files) would cause the newly
opened doc from being inserted into the old doc.

This approach is similar to Ace's sessions.

services/web/public/coffee/ide/editor/sharejs/vendor/client/cm.coffee

@@ -36,6 +36,8 @@ window.sharejs.extendDoc 'attach_cm', (editor, keepEditorContents) ->
     throw new Error 'Only text documents can be attached to CodeMirror2'
 
   sharedoc = @
+  editorDoc = editor.getDoc()
+
   check = ->
     window.setTimeout ->
         editorText = editor.getValue()
@@ -64,11 +66,10 @@ window.sharejs.extendDoc 'attach_cm', (editor, keepEditorContents) ->
   # Listen for edits in CodeMirror.
   editorListener = (ed, change) ->
     return if suppress
-    applyCMToShareJS editor, change, sharedoc
-
+    applyCMToShareJS editorDoc, change, sharedoc
     check()
 
-  editor.on 'change', editorListener
+  editorDoc.on 'change', editorListener
 
   @on 'insert', (pos, text) ->
     suppress = true
@@ -87,7 +88,7 @@ window.sharejs.extendDoc 'attach_cm', (editor, keepEditorContents) ->
 
   @detach_cm = ->
     # TODO: can we remove the insert and delete event callbacks?
-    editor.off 'onChange', editorListener
+    editorDoc.off 'change', editorListener
     delete @detach_cm
 
   return