Replace experimental path walk feature with upstream version (#5689)

This reverts the experimental version of the `git repack --path-walk`
feature and replaces it with the version that recently merged with
upstream. It includes `fixup!` commits for the reverts of the previous
feature for easy reduction in the branch thicket during the 2.51.0
release window.

> Note: In the current version, I have not performed any local
validation, only resolved one text merge conflict.

Author: dscho