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

[Ivan Shmakov]

>>>>> Stefan Monnier <monnier@IRO.UMontreal.CA> writes:

 >> 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 agree with Eli that it's a somewhat obscure corner case,

        (Which makes me wonder if using write-file on a tar-mode buffer
        a similarly obscure corner case?)

 > but as someone who does `M-x cd' fairly often for similar purposes, I
 > think the patch is right.

        Avoiding switching buffers until around the actual write-region
        call may still be a better alternative (as that should make
        mistakes of this kind hardier to introduce), but results in a
        lengthier patch.  Any opinion on which way I should go there?

        Unless there be objections, I hope to push a fix for this issue
        within the next two days.

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