pack-objects: zero-initialize tree_depth/layer arrays

pack-objects: zero-initialize tree_depth/layer arrays

Commit 108f530385 (pack-objects: move tree_depth into 'struct
packing_data', 2018-08-16) started maintaining a tree_depth array that
matches the "objects" array. We extend the array when:

  1. The objects array is extended, in which case we use realloc to
     extend the tree_depth array.

  2. A caller asks to store a tree_depth for object N, and this is the
     first such request; we create the array from scratch and store the
     value for N.

In the latter case, though, we use regular xmalloc(), and the depth
values for any objects besides N is undefined. This happens to not
trigger a bug with the current code, but the reasons are quite subtle:

 - we never ask about the depth for any object with index i < N. This is
   because we store the depth immediately for all trees and blobs. So
   any such "i" must be a non-tree, and therefore we will never need to
   care about its depth (in fact, we really only care about the depth of
   trees).

 - there are no objects at this point with index i > N, because we
   always fill in the depth for a tree immediately after its object
   entry is created (we may still allocate uninitialized depth entries,
   but they'll be initialized by packlist_alloc() when it initializes
   the entry in the "objects" array).

So it works, but only by chance. To be defensive, let's zero the array,
which matches the "unset" values which would be handed out by
oe_tree_depth() already.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>