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/Upgrade.hs

@@ -12,9 +12,9 @@ import Command
 import Upgrade
 import Annex.Version
 
-command :: [Command]
-command = [Command "upgrade" paramNothing noChecks seek
-	"upgrade repository layout"]
+def :: [Command]
+def = [dontCheck repoExists $ -- because an old version may not seem to exist
+	command "upgrade" paramNothing seek "upgrade repository layout"]
 
 seek :: [CommandSeek]
 seek = [withNothing start]