Merge branch 'tb/unsafe-hash-cleanup'

The API around choosing to use unsafe variant of SHA-1
implementation has been updated in an attempt to make it harder to
abuse.

* tb/unsafe-hash-cleanup:
  hash.h: drop unsafe_ function variants
  csum-file: introduce hashfile_checkpoint_init()
  t/helper/test-hash.c: use unsafe_hash_algo()
  csum-file.c: use unsafe_hash_algo()
  hash.h: introduce `unsafe_hash_algo()`
  csum-file.c: extract algop from hashfile_checksum_valid()
  csum-file: store the hash algorithm as a struct field
  t/helper/test-tool: implement sha1-unsafe helper

diff --cc builtin/fast-import.c
Simple merge

diff --cc bulk-checkin.c
Simple merge

diff --cc object-file.c
Simple merge

diff --cc t/helper/test-tool.c
Simple merge

diff --cc t/helper/test-tool.h
Simple merge