[PATCH 0/4] repack: various corner cases for cruft-less MIDXs

This patch series fixes a few bugs I spotted while investigating the cruft-less MIDX feature.

The bugs addressed are found in various corner cases, and, when triggered, may result in a MIDX being written whose objects are not closed under reachability. When this happens while the caller is trying to write reachability bitmaps, bitmap generation may fail if one or more selected commits are descendants of the open portion of the MIDX.

The series is structured as follows:

 * The first patch is a preparatory refactoring to add a context struct
   within pack-objects' handling of '--stdin-packs' to minimize the diff
   in the subsequent patch.

 * The second patch fixes a case where once-cruft tree and annotated tag
   objects may prevent reachability closure when objects reachable from
   them are not present in the input pack(s).

 * The third patch fixes a case where incremental repack operations may
   introduce the same bug when the pack generated by an incremental
   repack does not pack an additional copy of once-cruft object(s).

 * The fourth and final patch addresses a similar case involving .keep
   packs.

Thanks in advance for reviewing!

Taylor Blau (4):
  pack-objects: introduce `stdin_packs_context` struct
  pack-objects: ensure tree/tag closure with '--stdin-packs=follow'
  repack: retain cruft packs in MIDXs after incremental repacks
  repack: retain cruft packs in MIDXs containing kept packs

 Documentation/git-pack-objects.adoc |  2 +
 builtin/pack-objects.c              | 73 ++++++++++++++++++++------
 builtin/repack.c                    |  6 +++
 repack-midx.c                       |  5 ++
 t/t5331-pack-objects-stdin.sh       | 81 +++++++++++++++++++++++++++++
 t/t7704-repack-cruft.sh             | 51 ++++++++++++++++++
 6 files changed, 202 insertions(+), 16 deletions(-)

base-commit: a018953688f1b10bddf91bff8747068f5f4746a4