This commits adds rudimentary support for a cache directory, enabled by
default. The cache directory is created if it does not exist. The cache
is used if there's anything in it, newly created snapshot and index
files are written to the cache automatically.
The code now bundles tree blobs and data blobs into different pack
files, so we'll end up with pack files that either only contain data or
trees. This is in preparation to adding a cache (#1040), because
tree-only pack files can easily be cached later on.
