When a user presses undo it also undoes changes made by the other user.

Steps:

1. User A types something
2. User B types something
3. User A press undo

Expected behaviour (as seen in ACE):

User A's change is undone.

Actual behaviour:

User A and User B's changes are undone.

CC'ing the monaco guys:
@zach-karat @Progyan1997