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