Add `numberOfPackFilesAfterBitmap` to RepoStatistics

Introduce a `numberOfPackFilesAfterBitmap` that contains the number of
packfiles created since the latest bitmap generation.

Notes:
* the `repo.getObjectDatabase().getPacks()` that obtains the list of
  packs (in the existing `getStatistics` function) uses
  `PackDirectory.scanPacks` that boils down to call
  `PackDirectory.scanPacksImpl` which is sorting packs prior returning
  them therefore the `numberOfPackFilesAfterBitmap` is just all packs
  before the one that has bitmap attached
* the improved version of `packAndPrune` function (one that skips
  non-existent packfiles) was introduced for testing

Change-Id: I608011462f104fc002ac527aa405f492a8a4b0c2