All command line stuff today..

Added --want-get and --want-drop, which can be used to test preferred content settings of a repository. For example git annex find --in . --want-drop will list the same files that git annex drop --auto would try to drop. (Also renamed git annex content to git annex wanted.)

Finally laid to rest problems with git annex unannex when multiple files point to the same key. It's a lot slower, but I'll stop getting bug reports about that.