refs/files: remove empty parent dirs when aborting transactions

Hi,

this issue is something we recently discovered in Gitaly. It's nothing world breaking, but I think it makes sense to try and keep the refdb in a as-clean-as-possible state anyway.

Thanks!

Patrick

To: git@vger.kernel.org

--- b4-submit-tracking ---

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{ "series": { "revision": 1, "change-id": "20250708-b4-pks-reffiles-prune-empty-dirs-on-abort-15c7a7f279bd", "prefixes": [] } }

Closes Git leaks empty reference directories if refere... (#559 - closed).