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/Unannex.hs

@@ -17,9 +17,8 @@ import Annex.Content
 import qualified Git
 import qualified Git.LsFiles as LsFiles
 
-command :: [Command]
-command = [Command "unannex" paramPaths defaultChecks seek
-	"undo accidential add command"]
+def :: [Command]
+def = [command "unannex" paramPaths seek "undo accidential add command"]
 
 seek :: [CommandSeek]
 seek = [withFilesInGit start]