* lisp/tar-mode.el (tar-extract): Call tar--try-jka-compr (bug#34251)

* lisp/tar-mode.el (tar-extract): Call tar--try-jka-compr (bug#34251)

* lisp/tar-mode.el (tar--try-jka-compr): New function copied from
archive-try-jka-compr.

* lisp/arc-mode.el (archive-try-jka-compr): Set buffer-multibyte to t
instead of let-binding coding-system-for-read to 'no-conversion.

* test/data/decompress/tg.tar.gz:
* test/data/decompress/zg.zip: New fixtures.

* test/lisp/arc-mode-tests.el (arc-mode-test-zip-extract-gz):
* test/lisp/tar-mode-tests.el (tar-mode-test-tar-extract-gz): New tests.

* test/lisp/vc/diff-mode-tests.el (diff-mode-test-font-lock)
(diff-mode-test-font-lock-syntax-one-line): Skip unless shell and
diff executables are found.