fix t5000-tar-tree.sh when $TAR isn't set

$TAR isn't set everywhere. Provide a default (tar)

Signed-off-by: Alex Riesen <raa.lkml@gmail.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>

t/t5000-tar-tree.sh

@@ -25,6 +25,7 @@ commit id embedding:
 '
 
 . ./test-lib.sh
+TAR=${TAR:-tar}
 
 test_expect_success \
     'populate workdir' \