If ${REPODIR}/${PKG_ABI} does not exist when we begin real-update-packages,
skip the comparison with the non-existent previous repository and just
finish the repo off. This allows external scripts to just assume they can
run update-packages rather than figuring out if they'd previously run
packages for this Version/Arch combo.
PKG_VERSION_FROM_DIR was added so that we could perhaps detect the three
distinct cases:
1.) If the repo has not yet been created, PKG_VERSION_FROM_DIR will be
empty.
2.) If the repo is in some intermediate state between created and fully
initialized, PKG_VERSION_FROM_DIR may point to the ABI directory.
3.) If the repo is fully initialized, then PKG_VERSION_FROM_DIR points to
the latest build to compare to.
Option #2 is explicitly unhandled at the moment, but this is no different
than it was before.