build-export: scan repo for hardlinks on commit

This is a no-op on regular (archive) repos, but on bare repos it is
an optimization if the source is a checkout from the repo. This
happens in flatpak-builder (https://github.com/flatpak/flatpak-builder/pull/81)
when it commits the build to the cache during --install without --repo.

Closes: #1249
Approved by: alexlarsson
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f