Merge branch 'lp/repack-propagate-promisor-debugging-info' into seen

Merge branch 'lp/repack-propagate-promisor-debugging-info' into seen

When fetching objects into a lazily cloned repository, .promisor
files are created with information meant to help debugging.  "git
repack" has been taught to carry this information forward to
packfiles that are newly created.

* lp/repack-propagate-promisor-debugging-info:
  repack-promisor: add missing headers
  t7703: test for promisor file content after geometric repack
  t7700: test for promisor file content after repack
  repack-promisor: preserve content of promisor files after repack
  repack-promisor add helper to fill promisor file after repack
  pack-write: add explanation to promisor file content