Otherwise use the vendored copy as before.

The library is in Debian testing but not stable. Once it reaches
stable, the vendored copy can be removed.

Did not add it to debian/control because IIRC that's used to build
git-annex on stable too, possibly. However, the Debian maintainer will
probably want to make the package depend on libghc-git-lfs-dev.

This commit was sponsored by Ilya Shlyakhter on Patreon.