project-disk-use: skip percent utilization unless -d objects

project-disk-use: skip percent utilization unless -d objects

If repositories are set up in an unusual configuration where
multiple repositories share the same objects directory via
symbolic links, we skip using the reposizek value from that
repository as it will incorrectly skew the results.  Instead
the objects directory will be considered only with the
repository to which it really belongs.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>