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/Fix.hs

@@ -12,8 +12,8 @@ import Command
 import qualified Annex.Queue
 import Annex.Content
 
-command :: [Command]
-command = [Command "fix" paramPaths defaultChecks seek
+def :: [Command]
+def = [command "fix" paramPaths seek
 	"fix up symlinks to point to annexed content"]
 
 seek :: [CommandSeek]