add a UUID type

Should have done this a long time ago.

Logs/Location.hs

@@ -29,16 +29,15 @@ import Logs.Presence
 
 {- Log a change in the presence of a key's value in a repository. -}
 logChange :: Git.Repo -> Key -> UUID -> LogStatus -> Annex ()
-logChange repo key u s
-	| null u = error $
-		"unknown UUID for " ++ Git.repoDescribe repo ++ 
-		" (have you run git annex init there?)"
-	| otherwise = addLog (logFile key) =<< logNow s u
+logChange _ key (UUID u) s = addLog (logFile key) =<< logNow s u
+logChange repo _ NoUUID _ = error $
+	"unknown UUID for " ++ Git.repoDescribe repo ++ 
+	" (have you run git annex init there?)"
 
 {- Returns a list of repository UUIDs that, according to the log, have
  - the value of a key. -}
 keyLocations :: Key -> Annex [UUID]
-keyLocations = currentLog . logFile
+keyLocations key = map read <$> (currentLog . logFile) key
 
 {- Finds all keys that have location log information.
  - (There may be duplicate keys in the list.) -}