boost tarball

[Stefan Monnier <monnier@iro.umontreal.ca>]

Those people who tried to debug the boost-tarball problem recently
(or those people who tried to open large tarballs in Emacs) know that
Emacs is unusable on such large tarballs.

Well... was.

I've just installed a patch to tar-mode.el which makes it use the new
buffer-swap-text primitive.  So now the tar data is kept in an auxiliary
buffer rather than being mixed with the summary buffer.  This way we
don't need to mess with set-buffer-multibyte any more and the result is
that opening the 160MB boost.tar file now takes up just below 7s
(including starting Emacs) with a warm cache.  As opposed to more than
3 minutes before (most of it spent in set-buffer-multibyte after the
tar ball has already been loaded and parsed).

This is a significant change, so it's likely I've introduced bugs along
the way.


        Stefan