Add numberOfObjectsSinceBitmap to RepoStatistics

Introduce a numberOfObjectsSinceBitmap that contains the number of
objects stored in pack files and as loose objects created since the
latest bitmap generation.

Note that the existing
GcNumberOfPackFilesAfterBitmapStatisticsTest.java was renamed to
GcSinceBitmapStatisticsTest.java and extended to cover also this
statistic.

Change-Id: I8ae1db142ddfcd42a5a1d6da01bc67f695562e0e