Merge branch 'bc/sha1-256-interop-01'

Merge branch 'bc/sha1-256-interop-01'

The beginning of SHA1-SHA256 interoperability work.

* bc/sha1-256-interop-01:
  t1010: use BROKEN_OBJECTS prerequisite
  t: allow specifying compatibility hash
  fsck: consider gpgsig headers expected in tags
  rev-parse: allow printing compatibility hash
  docs: add documentation for loose objects
  docs: improve ambiguous areas of pack format documentation
  docs: reflect actual double signature for tags
  docs: update offset order for pack index v3
  docs: update pack index v3 format