cmd_diff(): use an object_array for holding trees

cmd_diff(): use an object_array for holding trees

Change cmd_diff() to use a (struct object_array) for holding the trees
that it accumulates, rather than rolling its own equivalent.

Incidentally, this change removes a hard-coded limit of 100 trees in
combined diff, not that it matters in practice.

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