In git.Clean use 'clean -fd' to clean directories as well as files
Implement time based repo locking

* Remove --pull flag
* Implemnt cache for repos based on last pull time
* A repo that has not been pulled in the last hour will be pulled
* The timestamps are stored system wide in /tmp/depman.cache
* Use --skip-cache to skip the cache once, --clear-cache to delete and
* recreate the cache