vi: clear undo buffer when we change to another file

vi: clear undo buffer when we change to another file

function                                             old     new   delta
init_text_buffer                                     156     190     +34
undo_push                                            360     382     +22
count_lines                                           74      72      -2
undo_pop                                             246     222     -24
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 2/2 up/down: 56/-26)             Total: 30 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>