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

[Stefan Monnier]

>       As currently implemented (tested on 7eb2077f9dd1, 2015-02-06
>       15:01:12 UTC), tar-untar-buffer uses the default-directory value
>       for the “data” buffer (see tar-data-buffer), instead of the
>       (arguably much less obscure) one for the tar-mode buffer itself
>       (which could easily be changed by the user via M-x cd.)

I agre with Eli that it's a somewhat obscure corner case, but as someone
who does `M-x cd' fairly often for similar purposes, I think the patch
is right.


        Stefan