Merge branch 'cc/delta-islands'

A few issues in the implementation of "delta-islands" feature has
been corrected.

* cc/delta-islands:
pack-objects: fix off-by-one in delta-island tree-depth computation
pack-objects: zero-initialize tree_depth/layer arrays
pack-objects: fix tree_depth and layer invariants

diff --cc builtin/pack-objects.c
Simple merge

diff --cc git-compat-util.h
Simple merge

diff --cc pack-objects.h
Simple merge