"Sam Bostock via GitGitGadget" [off-list ref] writes:

From: Sam Bostock <redacted>

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.

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"

It may be correct but I do not see a relevance

and never previously read config,
the global variables core_apply_sparse_checkout and
core_sparse_checkout_cone remained unset,

This may be correct, but it only becomes relevant after somebody
decides to do something to cause is_sparse_index_allowed() to say
Yes.

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 total, while individual sentences in the above may tell correct
things, the order of presentation makes it hard to understand, at
least to me.

The usual way to compose a log message of this project is to

 - Give an observation on how the current system works in the
   present tense (so no need to say "Currently X is Y", or
   "Previously X was Y" to describe the state before your change;
   just "X is Y" is enough), and discuss what you perceive as a
   problem in it.

 - Propose a solution (optional---often, problem description
   trivially leads to an obvious solution in reader's minds).

 - Give commands to somebody editing the codebase to "make it so",
   instead of saying "This commit does X".

in this order.

So perhaps

    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 sparse index, but because of the start
    up sequence of this command does not take necessary steps, we
    end up first expanding the index fully before doing this
    comparison.

    In order to convince sparse-index.c:is_sparse_index_allowed() to
    return true, we need to:

    - enable the global switch "core_apply_sparse_checkout" via
      the core.sparsecheckout configuration variable.  merge-ours
      currently do not even read basic configuration, so we need to
      make the configuration call ourselves.

    - set command_requires_full_index to 0.

    With that, the command can work without expanding the index
    fully before doing its work.

or something.

Thanks.