CLI/git: cache git indices

If the private index file matches a previously known revision of the
database, we can update the index incrementally using the recorded
lastmod counter. This is typically much faster than a full update,
although it could be slower in the case of large changes to the
database.

The "git-read-tree HEAD" is also a bottleneck, but unfortunately
sometimes is needed. Cache the index checksum and hash to reduce the
number of times the operation is run. The overall design is a
simplified version of the PrivateIndex class.