t7004: rename ULIMIT test prerequisite to ULIMIT_STACK_SIZE

During creation of the patch series, our discussion revealed that
we could have a more descriptive name for the prerequisite for the
test so it stays unique when other limits of ulimit are introduced.

Let's rename the existing ulimit about setting the stack size to
a more explicit ULIMIT_STACK_SIZE.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7004-tag.sh

@@ -1463,10 +1463,10 @@ run_with_limited_stack () {
 	(ulimit -s 128 && "$@")
 }
 
-test_lazy_prereq ULIMIT 'run_with_limited_stack true'
+test_lazy_prereq ULIMIT_STACK_SIZE 'run_with_limited_stack true'
 
 # we require ulimit, this excludes Windows
-test_expect_success ULIMIT '--contains works in a deep repo' '
+test_expect_success ULIMIT_STACK_SIZE '--contains works in a deep repo' '
 	>expect &&
 	i=1 &&
 	while test $i -lt 8000