Merge branch 'ab/object-file-api-updates'

Merge branch 'ab/object-file-api-updates'

Object-file API shuffling.

* ab/object-file-api-updates:
  object-file API: pass an enum to read_object_with_reference()
  object-file.c: add a literal version of write_object_file_prepare()
  object-file API: have hash_object_file() take "enum object_type"
  object API: rename hash_object_file_literally() to write_*()
  object-file API: split up and simplify check_object_signature()
  object API users + docs: check <0, not !0 with check_object_signature()
  object API docs: move check_object_signature() docs to cache.h
  object API: correct "buf" v.s. "map" mismatch in *.c and *.h
  object-file API: have write_object_file() take "enum object_type"
  object-file API: add a format_object_header() function
  object-file API: return "void", not "int" from hash_object_file()
  object-file.c: split up declaration of unrelated variables