[PATCH v2 0/2] merge-ours: sparse-index integration

This short series teaches merge-ours to work with a sparse index as a small step toward broader sparse-index support.

Patch 1 is a preparatory cleanup that converts merge-ours away from the_repository global, using the repo parameter instead.

Patch 2 adds the actual sparse-index integration and tests.

Changes since v1:

 * Patch 1: note in commit message that RUN_SETUP guarantees repo is never
   NULL (Patrick, Junio)
 * Patch 2: rewrite commit message to follow the project's standard log
   message structure (Junio)

Thanks Junio and Patrick for the review.

Developed with AI assistance (Claude).

Sam Bostock (2):
  merge-ours: drop USE_THE_REPOSITORY_VARIABLE
  merge-ours: integrate with sparse-index

 builtin/merge-ours.c                     | 15 +++++++++------
 t/t1092-sparse-checkout-compatibility.sh | 14 ++++++++++++++
 2 files changed, 23 insertions(+), 6 deletions(-)

base-commit: b2826b52eb7caff9f4ed6e85ec45e338bf02ad09
Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-git-2189%2Fsambostock%2Fsb%2Fmerge-ours-sparse-index-v2
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-git-2189/sambostock/sb/merge-ours-sparse-index-v2
Pull-Request: https://github.com/git/git/pull/2189

Range-diff vs v1:

 1:  6cb970e512 ! 1:  775d76df69 merge-ours: drop USE_THE_REPOSITORY_VARIABLE
     @@ Metadata
       ## Commit message ##
          merge-ours: drop USE_THE_REPOSITORY_VARIABLE
      
     -    Use the `repo` parameter passed to cmd_merge_ours() instead of
     -    `the_repository`, and drop the USE_THE_REPOSITORY_VARIABLE macro that
     -    is no longer needed.
     +    The merge-ours built-in uses the `the_repository` global to access
     +    the repository. The project is moving away from this global in favor
     +    of the `repo` parameter that is passed to each built-in command.
     +    Since merge-ours is registered with RUN_SETUP, `repo` is guaranteed
     +    to be non-NULL and can be used directly.
     +
     +    Drop the USE_THE_REPOSITORY_VARIABLE macro and use `repo` throughout.
      
          While at it, remove a stray double blank line between the #include
          block and the usage string.
 2:  20b9e0bf6e ! 2:  55d39ff778 merge-ours: integrate with sparse-index
     @@ Metadata
       ## Commit message ##
          merge-ours: integrate with sparse-index
      
     -    The merge-ours builtin reads the index only to compare it against HEAD
     -    via index_differs_from(), whose diff machinery (run_diff_index) is
     -    already sparse-aware.
     +    The merge-ours built-in opens the index to compare it against HEAD.
     +    The machinery used to do this (i.e. run_diff_index()) is capable of
     +    working with a sparse index, but the start-up sequence of this
     +    command does not take the necessary steps, so we end up expanding the
     +    index fully before doing the comparison.
      
     -    Teach merge-ours to opt out of requiring a full index by setting
     -    command_requires_full_index to 0. Because merge-ours is invoked as a
     -    subprocess by "git merge -s ours" and never previously read config,
     -    the global variables core_apply_sparse_checkout and
     -    core_sparse_checkout_cone remained unset, causing
     -    is_sparse_index_allowed() to return false and the index to be expanded
     -    anyway. Add a repo_config() call with git_default_config to populate
     -    these globals.
     +    In order to convince sparse-index.c:is_sparse_index_allowed() to
     +    return true, we need to:
      
     -    Add tests to t1092 verifying that "git merge -s ours" produces
     -    identical results across full-checkout, sparse-checkout, and
     -    sparse-index modes, including verifying the resulting merge commit
     -    structure, and that the sparse index is not expanded during the
     -    operation.
     +     - Read basic configuration with git_default_config so that global
     +       variables like core_apply_sparse_checkout are populated.
     +       merge-ours currently does not read configuration at all.
     +
     +     - Set command_requires_full_index to 0.
     +
     +    With that, the command can work without expanding the index fully
     +    before doing its work.
      
          Signed-off-by: Sam Bostock <sam@sambostock.ca>