Graph caching must hold copies of cache objects instead of giving
them away for free and risk of having them polluted or even completely
cleared out under its own chair.
Example:
$ equo remove linux-sabayon:3.1 [--deep]
The --deep version doesn't pull in all the reverse deps due to
Graph.solve() being called twice in
entropy.client.interfaces.dep's _generate_reverse_dependency_tree()
when deep = True.
Fixes bug #2989