bug#19865: tar-untar-buffer: should honor default-directory

[Stefan Monnier]

>       Per my reading of the code, with-current-buffer is generally
>       used in tar-mode.el around the smallest fragments possible.

Indeed.

>       Please thus consider the revised patch MIMEd, which I’ve tried
>       to make consistent with such an approach.

I think it's better, indeed.

BTW, the (set-buffer-multibyte nil) should probably just be remove (or
replaced by a cl-assert)


        Stefan