t: use configured TAR instead of tar

t: use configured TAR instead of tar

Despite that tar is available everywhere, it's not required by POSIX.

In our build system, users are allowed to specify which tar to be used
in Makefile knobs. Furthermore, GNU tar (gtar) is prefered when autotools
is being used.

In our testsuite, 7 out of 9 tar-required-tests use "$TAR", the other
two use "tar".

Let's change the remaining two tests to "$TAR".

Signed-off-by: Đoàn Trần Công Danh <congdanhqx@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>