Merge branch 'jk/oi-delta-base'

Teach "cat-file --batch" to show delta-base object name for a
packed object that is represented as a delta.

* jk/oi-delta-base:
  cat-file: provide %(deltabase) batch format
  sha1_object_info_extended: provide delta base sha1s

diff --cc builtin/cat-file.c
Simple merge

diff --cc cache.h
Simple merge

diff --cc sha1_file.c
Simple merge

diff --cc t/t1006-cat-file.sh
Simple merge