Allow to get repo statistics from GarbageCollectionCommand before gc

Allow to get repo statistics from GarbageCollectionCommand before gc

When running the garbage collection for a repository it is often
interesting to compare the repository statistics from before and after
the garbage collection to understand the effect of the garbage
collection. This is why it makes sense that the
GarbageCollectionCommand provides a method to retrieve the repository
statistics before running the garbage collection.

So far without running the garbage collection the repository statistics
can only be retrieved by using JGit internal classes. This is what EGit
and Gerrit do at the moment, but it would be better to have an API for
this.

Change-Id: Id7e579157e9fbef5cfd1fc9f97ada45f0ca8c379
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>