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

[Ivan Shmakov]

        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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