Merge branch 'bc/submodule-force-same-hash'

Adding a repository that uses a different hash function is a no-no,
but "git submodule add" did nt prevent it, which has been corrected.

* bc/submodule-force-same-hash:
  read-cache: drop submodule check from add_to_cache()
  object-file: disallow adding submodules of different hash algo

diff --cc object-file.c
Simple merge

diff --cc read-cache.c
Simple merge