FIX: assertion prevents from (allowed) double Traverse()
add scaffold for `cvmfs_swissknife gc`
factor out MockCatalog for code reuse
add mocked files to MockCatalog
add file chunk mocks to MockCatalog
TEST: scaffold for garbage collector test case
add a simple implementation of a HashFilter
TEST: basic test case for SimpleHashFilter
remove copy and paste garbage
add implementation of GarbageDetector
make all MockCatalogs directly accessible for the test cases
add MockCatalog::ListReferencedObjects()
allow for counting objects in HashFilter
instrument SimpleHashFilter for testability
TEST: first simple garbage collection test case
TEST: T_GarbageCollector.PreserveLatestTwoRevisions
mv gc*.h garbage_detector*.h
add include guard around hash_filter.h
typedef for CatalogT in CatalogTraversal<>
extend logging facility for GC