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

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

diff --cc Documentation/Makefile
Simple merge

diff --cc Documentation/fsck-msgids.adoc
Simple merge

diff --cc Documentation/meson.build
Simple merge

diff --cc builtin/rev-parse.c
Simple merge

diff --cc fsck.h
Simple merge

diff --cc t/test-lib.sh
Simple merge