Accidental clicks on the revert hunk/lines buttons can cause loss of
work, and can be frustrating. So, allow undoing the last revert.
Right now, a stack or deque are not being used for the sake of
simplicity, so only one undo is possible. Any reverts before the
previous one are lost.
Signed-off-by: Pratyush Yadav <me@yadavpratyush.com>