t5616: use rev-parse instead to get HEAD's object_id

Only HEAD's object_id is necessary, rev-list is an overkill.

Despite POSIX requires grep(1) treat single pattern with <newline>
as multiple patterns.
busybox's grep(1) (as of v1.31.1) haven't implemented it yet.

Use rev-parse to simplify the test and avoid busybox unimplemented
features.

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

t/t5616-partial-clone.sh

@@ -49,7 +49,7 @@ test_expect_success 'do partial clone 1' '
 test_expect_success 'verify that .promisor file contains refs fetched' '
 	ls pc1/.git/objects/pack/pack-*.promisor >promisorlist &&
 	test_line_count = 1 promisorlist &&
-	git -C srv.bare rev-list HEAD >headhash &&
+	git -C srv.bare rev-parse --verify HEAD >headhash &&
 	grep "$(cat headhash) HEAD" $(cat promisorlist) &&
 	grep "$(cat headhash) refs/heads/master" $(cat promisorlist)
 '