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 <c24c152f145c57791e809b126db9e61bd1c1782d@alum.mit.edu>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git