Merge branch 'bc/object-id'

Identify parts of the code that knows that we use SHA-1 hash to
name our objects too much, and use (1) symbolic constants instead
of hardcoded 20 as byte count and/or (2) use struct object_id
instead of unsigned char [20] for object names.

* bc/object-id:
  apply: convert threeway_stage to object_id
  patch-id: convert to use struct object_id
  commit: convert parts to struct object_id
  diff: convert struct combine_diff_path to object_id
  bulk-checkin.c: convert to use struct object_id
  zip: use GIT_SHA1_HEXSZ for trailers
  archive.c: convert to use struct object_id
  bisect.c: convert leaf functions to use struct object_id
  define utility functions for object IDs
  define a structure for object IDs

diff --cc archive-zip.c
Simple merge

diff --cc archive.c
Simple merge

diff --cc builtin/apply.c
Simple merge

diff --cc bulk-checkin.c
Simple merge

diff --cc cache.h
Simple merge

diff --cc commit.h
Simple merge

diff --cc diff-lib.c
Simple merge

diff --cc log-tree.c
Simple merge

diff --cc send-pack.c
Simple merge

diff --cc shallow.c
Simple merge

diff --cc upload-pack.c
Simple merge