Populating the note after opening the split should reset the undo history

[m1foley]

To recreate:

1. :SimplenoteList
2. Press Return to open a note. The cursor is now in a new split, which has been populated with the note data.
3. Press u

Expected behavior: Nothing should happen.
Actual behavior: It clears the buffer.

I experienced this after pressing u a few times to undo my changes to the note. My intention was to restore the original note, but the behavior caused me to worry that the note would be erased.