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/Map.hs

@@ -22,9 +22,9 @@ import qualified Utility.Dot as Dot
 -- a link from the first repository to the second (its remote)
 data Link = Link Git.Repo Git.Repo
 
-command :: [Command]
-command = [Command "map" paramNothing noChecks seek
-	"generate map of repositories"]
+def :: [Command]
+def = [dontCheck repoExists $
+	command "map" paramNothing seek "generate map of repositories"]
 
 seek :: [CommandSeek]
 seek = [withNothing start]