clean up check selection code

This new approach allows filtering out checks from the default set that are
not appropriate for a command, rather than having to list every check
that is appropriate. It also reduces some boilerplate.

Haskell does not define Eq for functions, so I had to go a long way around
with each check having a unique id. Meh.

Command/Unused.hs

@@ -27,8 +27,8 @@ import qualified Remote
 import qualified Annex.Branch
 import Annex.CatFile
 
-command :: [Command]
-command = [Command "unused" paramNothing (noTo >> needsRepo) seek
+def :: [Command]
+def = [dontCheck fromOpt $ command "unused" paramNothing seek
 	"look for unused file content"]
 
 seek :: [CommandSeek]