gpgtar: Make option -C work for archive creation.

* tools/gpgtar-create.c (gpgtar_create): Switch to the -C directory.

The -C option is pretty useful given that pattern are always relative
to the current directory. In contrast to GNU tar, the switching is
done only once.

(cherry picked from commit b3a7a5140784b5a015107b5c5c73b15ae44e71dc)

• Signed-off-by: Werner Koch <wk@gnupg.org>