The global cache only grows and there is no way for the darcs client to
clean it. We need to introduce a command that deletes unused files from
the global cache.
Implement a command “darcs optimize –global-cache” that scans the home
directory (or some folder given as argument) to find repositories,
garbage collect them (optionally?), and only keep hashed files that
correspond to these repositories.
See <http://darcs.net/Internals/CacheSystem>