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/Uninit.hs

@@ -18,8 +18,8 @@ import Init
 import qualified Annex.Branch
 import Annex.Content
 
-command :: [Command]
-command = [Command "uninit" paramPaths (check >> defaultChecks) seek 
+def :: [Command]
+def = [addCheck check $ command "uninit" paramPaths seek 
         "de-initialize git-annex and clean out repository"]
 
 check :: Annex ()