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/Version.hs

@@ -12,8 +12,9 @@ import Command
 import qualified Build.SysConfig as SysConfig
 import Annex.Version
 
-command :: [Command]
-command = [Command "version" paramNothing noChecks seek "show version info"]
+def :: [Command]
+def = [dontCheck repoExists $
+	command "version" paramNothing seek "show version info"]
 
 seek :: [CommandSeek]
 seek = [withNothing start]