Two last-minute changes before 2.46.0

These two changes would be nice to have before rebasing onto 2.46.0.

Revert b7d6f23a17110d597d58f4a8e1b34b7a72c43fe1 due to the regression it introduced to Git 2.45.0 and the incremental-repack maintenance step. Since git pack-objects --stdin-packs uses a notion of "included" and "excluded" packs, it separates the choice to repack an object from whether or not the multi-pack-index points to the copy in the selected pack-files. Thus, .scalarCaches are filling up with pack-files with fewer than a dozen reference objects, but they still can't be expired with git multi-pack-index expire. This revert fixes the problem but will need more justification, testing, and consideration of the server-side experience when moving upstream.
Add an advice message around the sparse index expanding. This commit is already in next in upstream, but just missed the v2.46.0 release window. This advice will be valuable for microsoft/git customers, especially those using the Office monorepo. The commit was cherry-picked from next, but requires an additional change on top to make some microsoft/git-specific tests pass due to comparing stderr.

microsoft / git