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/Merge.hs

@@ -11,8 +11,8 @@ import Common.Annex
 import Command
 import qualified Annex.Branch
 
-command :: [Command]
-command = [Command "merge" paramNothing defaultChecks seek
+def :: [Command]
+def = [command "merge" paramNothing seek
 		"auto-merge remote changes into git-annex branch"]
 
 seek :: [CommandSeek]