process_{tree,blob}: show objects without buffering

Here's a less trivial thing, and slightly more dubious one.

I was looking at that "struct object_array objects", and wondering why we
do that. I have honestly totally forgotten. Why not just call the "show()"
function as we encounter the objects? Rather than add the objects to the
object_array, and then at the very end going through the array and doing a
'show' on all, just do things more incrementally.

Now, there are possible downsides to this:

 - the "buffer using object_array" _can_ in theory result in at least
   better I-cache usage (two tight loops rather than one more spread out
   one). I don't think this is a real issue, but in theory..

 - this _does_ change the order of the objects printed. Instead of doing a
   "process_tree(revs, commit->tree, &objects, NULL, "");" in the loop
   over the commits (which puts all the root trees _first_ in the object
   list, this patch just adds them to the list of pending objects, and
   then we'll traverse them in that order (and thus show each root tree
   object together with the objects we discover under it)

   I _think_ the new ordering actually makes more sense, but the object
   ordering is actually a subtle thing when it comes to packing
   efficiency, so any change in order is going to have implications for
   packing. Good or bad, I dunno.

 - There may be some reason why we did it that odd way with the object
   array, that I have simply forgotten.

Anyway, now that we don't buffer up the objects before showing them
that may actually result in lower memory usage during that whole
traverse_commit_list() phase.

This is seriously not very deeply tested. It makes sense to me, it seems
to pass all the tests, it looks ok, but...

Does anybody remember why we did that "object_array" thing? It used to be
an "object_list" a long long time ago, but got changed into the array due
to better memory usage patterns (those linked lists of obejcts are
horrible from a memory allocation standpoint). But I wonder why we didn't
do this back then. Maybe there's a reason for it.

Or maybe there _used_ to be a reason, and no longer is.

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