[GSoC PATCH v2 0/7] repack: add --drop-filtered to reclaim space in partial clones

This is v2 of the series adding "git repack --drop-filtered" to reclaim disk space in partial clones by dropping large, locally-held promisor blobs that remain recoverable from the promisor remote. v1 was sent as an RFC [1].

Partial clones let you work with large repositories without downloading every blob up front. Mising blobs are lazily fetched from the promisor remote on demand. Over time these accumulate locally and there is currently no safe, built-in way to reclaim that space short of re-cloning. This series adds that reverse direction: enumerate promisor blobs over a size threshold, drop them locally, and rely on the existing lazy-fetch machinery to bring them back transparently when needed.

How it works:
  * Enumerate promisor objects directly (ODB_FOR_EACH_OBJECT_PROMISOR_ONLY)
    and select the blobs exceeding the filter threshold. Every enumerated
    object is a promisor object by construction, so it is guaranteed
    recoverable and locally-created objects are never candidates.
  * Rebuild the promisor pack without the selected blobs, reusing the
    existing repack machinery, so the drop is crash-safe (write, fsync,
    install, then delete the old pack).
  * --dry-run lists the candidates and changes nothing.

Safety guards refuse to run while a merge, rebase, am, cherry-pick, revert, or bisect is in progress, and refuse to drop a blob referenced by the current index (it would only be lazily re-fetched by the next worktree command). Both are skipped for bare repositories.

Changes since v1:
  * distinguish an explicit -b/--write-bitmap-index on the command line
    (reported as a conflict) from a repack.writeBitmaps config value
    (silently disabled for the command). This addresses Junio's review
    that the previous check could not tell the two apart
  * documented the choice to keep --dry-run as a separate option rather
    than --drop-filtered=<mode>
  * implemented the safety guards
  * Added git-repack documentation for --drop-filtered and --dry-run
  * Reorganised so enumerate_promisor_blobs() is introduced in its final
    signature
  * Distributed the tests into the commits that introduce each behavior,
    instead of a single standalone test commit.
  * Dropped the drop-log commit from this series

To do:
  * Remote verification: verifying against the remote awaits the "remote-object-info"
    cat-file protocol command.
  * Drop log: introduce with the error-path change that reads it.
  * --verbose: space-reclaimed reporting.

[1] https://lore.kernel.org/git/20260716132848.95982-1-r.siddharth.shrimali@gmail.com/

Siddharth Shrimali (7):
  builtin/repack.c: add --drop-filtered and --dry-run options
  list-objects-filter: add list_objects_filter__filter_oidset()
  repack-promisor: allow excluding objects from the rebuilt promisor
    pack
  builtin/repack: enumerate promisor blobs for --drop-filtered
  builtin/repack: actually drop filtered promisor blobs
  builtin/repack: add safety guards for --drop-filtered
  Documentation/git-repack: document --drop-filtered and --dry-run

 Documentation/git-repack.adoc   |  35 +++++++
 builtin/repack.c                | 135 +++++++++++++++++++++++-
 list-objects-filter.c           |  45 ++++++++
 list-objects-filter.h           |  16 +++
 repack-filtered.c               |  81 +++++++++++++++
 repack-promisor.c               |  15 ++-
 repack.h                        |   8 +-
 t/meson.build                   |   1 +
 t/t7706-repack-drop-filtered.sh | 179 ++++++++++++++++++++++++++++++++
 9 files changed, 511 insertions(+), 4 deletions(-)
 create mode 100755 t/t7706-repack-drop-filtered.sh