save-buffer in tar-mode

[Tak Ota]

Recently I realized that undo information was discarded as soon as
executing save-buffer in a buffer which was opened from tar-mode.
tar-subfile-save-buffer at a glance didn't lead me to exactly where it
discarded undo information.  Anyway, is this an intended behavior?
Normal save-buffer does not invalidate undo information.  It is quite
annoying losing ability to undo in this way.  I'm using the head of
CVS.

-Tak