grep: avoid using oid_to_hex() with parse_object_or_die()

grep: avoid using oid_to_hex() with parse_object_or_die()

parse_object_or_die() is passed an object ID and a name to show if the
object cannot be parsed.  If the name is NULL then it shows the
hexadecimal object ID.  Use that feature instead of preparing and
passing the hexadecimal representation to the function proactively.
That's shorter and a bit more efficient.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>