Re: [PATCH v3 2/7] sparse-checkout: add basics of 'clean' command

>> +Some special cases, such as merge conflicts or modified files outside of
>> +the sparse-checkout definition could lead to keeping files that would
>> +otherwise be removed. Resolve conflicts, stage modifications, and use
>> +`git sparse-checkout reapply` in conjunction with `git sparse-checkout
>> +clean` to resolve these cases.
>> ++
>> +This command can be used to be sure the sparse index works efficiently,
>> +though it does not require enabling the sparse index feature via the
>> +`index.sparse=true` configuration.
> 
> This expanded explanation for users is nice too.  I particularly like
> that you called out three things users need to use in conjunction with
> this command -- resolving conflicts, staging modifications, and using
> `git sparse-checkout reapply`...

> [...]
>> +       if (convert_to_sparse(repo->index, SPARSE_INDEX_MEMORY_ONLY) ||
>> +           repo->index->sparse_index == INDEX_EXPANDED)
>> +               die(_("failed to convert index to a sparse index; resolve merge conflicts and try again"));
> 
> ...yet the error message you give to users only lists one of those
> three things even though the other two may be the problem.  Could we
> fix up the error message?