Fix so it works when given repo path is relative

Currently, if updaterepo.py is given a relative path, it will always re-add the same files because `packagesFoundOnDisk` will contain relative paths, but `packagesInDbKeys` will contain absolute paths so `packagesFoundOnDisk - packagesInDbKeys == packagesFoundOnDisk`.

With this PR, it will always work with absolute paths so the set operations work as expected.