More abstraction of hash function from the codepath.

* bc/hash-algo:
  hash: update obsolete reference to SHA1_HEADER
  bulk-checkin: abstract SHA-1 usage
  csum-file: abstract uses of SHA-1
  csum-file: rename sha1file to hashfile
  read-cache: abstract away uses of SHA-1
  pack-write: switch various SHA-1 values to abstract forms
  pack-check: convert various uses of SHA-1 to abstract forms
  fast-import: switch various uses of SHA-1 to the_hash_algo
  sha1_file: switch uses of SHA-1 to the_hash_algo
  builtin/unpack-objects: switch uses of SHA-1 to the_hash_algo
  builtin/index-pack: improve hash function abstraction
  hash: create union for hash context allocation
  hash: move SHA-1 macros to hash.h