v0.5.3

Speed load/merge/pull for large repositories using same mechanism as for sync, now we take into consideration just changed packages instead scan all packages in repository.

also:

• fix multi-level submodule directory
• fix #isAnsestorOf: (it was scanning all commits, now uses the correct backend revwalk)
• fix some types
• added "system repositories" concept, to keep pointer to pharo specific repositories (and do not show them by default).
• fix a problem when branches and not valid repositories