optimize: A new subcommand that either gets or drops file content as needed to work toward meeting the configured numcopies setting.

This is currently rather simplistic, though still useful.
In the future, it could become smarter about what content is stored where,
etc.

Command.hs

@@ -131,6 +131,8 @@ withAttrFilesInGit attr a params = do
 	repo <- Annex.gitRepo
 	files <- liftIO $ runPreserveOrder (LsFiles.inRepo repo) params
 	liftM (map a) $ liftIO $ Git.checkAttr repo attr files
+withNumCopies :: CommandSeekAttrFiles
+withNumCopies = withAttrFilesInGit "annex.numcopies"
 withBackendFilesInGit :: CommandSeekBackendFiles
 withBackendFilesInGit a params = do
 	repo <- Annex.gitRepo