reduce_heads(): thinkofix

reduce_heads(): thinkofix

When comparing two commit objects for equality, it is sufficient to
compare their in-core pointers because the object layer guarantees the
uniqueness.  However, comparing pointers to two "struct commit_list"
instances that point at the same commit does not make any sense.

Spotted by Sverre Hvammen Johansen who wrote an additional test to expose
the problem, fixed by Miklos Vajna.

Signed-off-by: Junio C Hamano <gitster@pobox.com>