git-update-dirs: compress: Only run "ga repair" if -d/--dangling is used

I believe the index corruption only happens if objects mentioned in
.git/annex/index are pruned from the repository, and that is done only
if -d/--dangling is used.

197f0e72-e07b-11e3-a7eb-fefdb24f8e10
5e6584a8-e07b-11e3-8212-fefdb24f8e10
git-update-dirs: Add -D/--delete-dangling option

73af80ee-e07f-11e3-93b7-c80aa9e67bbd