do workaround on replacing entire buffer not to move the cursor on undo (issue #8)

Replace current buffer with workaround not to move
the cursor on undo (issue #8)

The points are:
  - Do not touch the first line.
  - Use :put (p, P and :put! is not available).

To meet above condition:
  - Delete all lines except for the first line.
  - Put formatted text except for the first line.