Merge branch 'bc/hash-algo'

An infrastructure to define what hash function is used in Git is
introduced, and an effort to plumb that throughout various
codepaths has been started.

* bc/hash-algo:
  repository: fix a sparse 'using integer as NULL pointer' warning
  Switch empty tree and blob lookups to use hash abstraction
  Integrate hash algorithm support with repo setup
  Add structure representing hash algorithm
  setup: expose enumerated repo info

diff --cc builtin/am.c
Simple merge

diff --cc builtin/checkout.c
Simple merge

diff --cc builtin/pull.c
Simple merge

diff --cc cache.h
Simple merge

diff --cc diff-lib.c
Simple merge

diff --cc merge-recursive.c
Simple merge

diff --cc sequencer.c
Simple merge

diff --cc sha1_file.c
Simple merge

diff --cc submodule.c
Simple merge