Add pack options to preserve and prune old pack files

Add pack options to preserve and prune old pack files

Add the options
- pack.preserveOldPacks
- pack.prunePreserved

This allows to configure in git config if old packs should be preserved
during gc and pruned during the next gc.

The original implementation in 91132bb0 only allows to set these options
using the API.

Change-Id: I5b23ab4f317d12f5ccd234401419913e8263cc9a