WindowCache: add metric for cached bytes per repository

WindowCache: add metric for cached bytes per repository

Since ObjectDatabase and PackFile don't know their repository use the
packfile's grand-grand-parent directory as an identifier for the
repository the packfile resides in.

Remove metric for a repository if the number of cached bytes for the
repository drops to 0 in order to ensure the map of cached bytes per
repository doesn't contain repositories which have no data cached in the
WindowCache.

Change-Id: I969ab8029db0a292e7585cbb36ca0baa797da20b
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>