Suppress merging selections during undo/redo

Now, during undo/redo overlapping selections will be temporarily
created as markers are created via snapshots. Old selections will
immediately be destroyed though, since undo/redo now completely
replace all historied markers w/ those in the snapshot, so there
is no need to merge selections.

src/text-editor.coffee

@@ -1119,12 +1119,12 @@ class TextEditor extends Model
 
   # Essential: Undo the last change.
   undo: ->
-    @buffer.undo()
+    @avoidMergingSelections => @buffer.undo()
     @getLastSelection().autoscroll()
 
   # Essential: Redo the last change.
   redo: ->
-    @buffer.redo(this)
+    @avoidMergingSelections => @buffer.redo()
     @getLastSelection().autoscroll()
 
   # Extended: Batch multiple operations as a single undo/redo step.
@@ -2217,6 +2217,9 @@ class TextEditor extends Model
 
       previousSelection.intersectsScreenRowRange(screenRange.start.row, screenRange.end.row)
 
+  avoidMergingSelections: (args...) ->
+    @mergeSelections args..., -> false
+
   mergeSelections: (args...) ->
     mergePredicate = args.pop()
     fn = args.pop() if _.isFunction(_.last(args))