odb: add write_packfile, for_each_unique_abbrev, convert_object_id

[MayCXC]

This adds three ODB source vtable methods that were not part of the
recent ps/odb-sources and ps/object-counting series, plus caller
routing for object-name.c and fast-import.c.

New vtable methods:

• write_packfile: Ingest a pack from a file descriptor. The files
  backend chooses between index-pack (large packs) and
  unpack-objects (small packs below fetch.unpackLimit). Options
  cover thin-pack fixing, promisor marking, fsck, lockfile capture,
  and shallow file passing. Non-files backends can handle pack
  ingestion through their own mechanism.

• for_each_unique_abbrev: Iterate objects matching a hex prefix for
  disambiguation. The files backend searches loose objects via
  oidtree, multi-pack indices, then non-MIDX packs.

• convert_object_id: Translate between hash algorithms using the
  loose object map. Used during SHA-1 to SHA-256 migration.

Caller routing:

• object-name.c: The abbreviation and disambiguation paths
  (find_short_object_filename, find_abbrev_len_packed, and
  find_short_packed_object) directly access files-backend internals
  (loose cache, pack store, MIDX). These are converted to dispatch
  through the for_each_unique_abbrev vtable method, so that
  non-files backends participate through proper abstraction rather
  than being skipped.

• fast-import.c: end_packfile() replaced direct pack indexing,
  registration, and odb_source_files_downcast() with a call to
  odb_write_packfile(). gfi_unpack_entry() falls back to
  odb_read_object() when the pack slot is NULL (non-files backends
  ingest packs without registering them on disk).

This addresses Patrick's feedback on the previous submission [1]:
the correct fix for downcast sites is proper vtable abstraction, not
skipping non-files backends.

Additional:

• ODB_SOURCE_HELPER added to the source type enum
• odb/source-type.h extracted to avoid circular includes with
  repository.h
• OBJECT_INFO_KEPT_ONLY flag for backends that track kept status
• self_contained_out output field on odb_write_packfile_options

Motivation: These methods are needed by the local helper backend
series [2], which delegates object and reference storage
to external git-local- helper processes. sqlite-git [3] is a
working proof of concept that stores objects, refs, and reflogs
in a single SQLite database with full worktree support.

CC: Junio C Hamano gitster@pobox.com, Patrick Steinhardt ps@pks.im

[1] https://github.com/gitgitgadget/git/pull/2068.patch
[2] https://github.com/gitgitgadget/git/compare/master...MayCXC:git:ps/series-2-helpers-v3.patch
[3] https://github.com/MayCXC/sqlite-git

[gitgitgadget]

There is an issue in commit 3d511c2:
odb: add write_packfile, for_each_unique_abbrev, convert_object_id

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[MayCXC]

/submit

[gitgitgadget]

Submitted as pull.2074.git.1774530437562.gitgitgadget@gmail.com

To fetch this version into FETCH_HEAD:

git fetch https://github.com/gitgitgadget/git/ pr-2074/MayCXC/ps/series-1-vtable-v3-v1

To fetch this version to local tag pr-2074/MayCXC/ps/series-1-vtable-v3-v1:

git fetch --no-tags https://github.com/gitgitgadget/git/ tag pr-2074/MayCXC/ps/series-1-vtable-v3-v1

[MayCXC]

/submit

[gitgitgadget]

Submitted as pull.2074.v2.git.1774532383055.gitgitgadget@gmail.com

To fetch this version into FETCH_HEAD:

git fetch https://github.com/gitgitgadget/git/ pr-2074/MayCXC/ps/series-1-vtable-v3-v2

To fetch this version to local tag pr-2074/MayCXC/ps/series-1-vtable-v3-v2:

git fetch --no-tags https://github.com/gitgitgadget/git/ tag pr-2074/MayCXC/ps/series-1-vtable-v3-v2