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

[Ivan Shmakov <ivan@siamics.net>]

	Well, let’s see if I can reword the report.

	As currently implemented (tested on 7eb2077f9dd1, 2015-02-06
	15:01:12 UTC), the *user* tar-untar-buffer command uses the
	value of the default-directory *user* variable, as set for the
	*internal* (tar-data-buffer) buffer – in place of the value that
	same variable has in the buffer /the command is invoked from./

	To stress it out: this report is /not/ intended to be a request
	for a feature for unpacking Tar archives into arbitrary
	directories, but is rather meant to point out that as currently
	implemented, the tar-untar-buffer behavior is *ill-defined*;
	especially if used after write-file (C-x C-w), save-buffer
	(C-x C-s; when a Tar archive is created from scratch), or
	M-x cd RET.  (As all these commands /may/ change the value of
	default-directory for the user interaction buffer, but do /not/
	do the same for the *internal* tar-data-buffer one.)

	I believe that using the values the *user* variables posses in
	*internal* buffers, when the command in question is invoked from
	a buffer dedicated to the *user* interaction and implemented by
	that same facility, is generally a wrong thing to do, and has a
	potential for confusion of users and developers alike.

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