Merge branch 'bc/sha1-256-interop-02' into seen

The code to maintain mapping between object names in multiple hash
functions is being added, written in Rust.

* bc/sha1-256-interop-02:
  SQUASH??? downgrade build.rs syntax
  object-file-convert: always make sure object ID algo is valid
  rust: add a small wrapper around the hashfile code
  rust: add a new binary loose object map format
  rust: add functionality to hash an object
  rust: add a build.rs script for tests
  hash: expose hash context functions to Rust
  write-or-die: add an fsync component for the loose object map
  csum-file: define hashwrite's count as a uint32_t
  hash: add a function to look up hash algo structs
  rust: add a hash algorithm abstraction
  rust: add a ObjectID struct
  hash: use uint32_t for object_id algorithm
  conversion: don't crash when no destination algo
  repository: require Rust support for interoperability

diff --cc Makefile
Simple merge

diff --cc repository.h
Simple merge

diff --cc t/t1016-compatObjectFormat.sh
Simple merge