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

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 <redacted>
Signed-off-by: Junio C Hamano <redacted>