add a UUID type

Should have done this a long time ago.

Command/Map.hs

@@ -62,7 +62,7 @@ drawMap rs umap ts = Dot.graph $ repos ++ trusted ++ others
 		others = map (unreachable . uuidnode) $
 			filter (`notElem` ruuids) (M.keys umap)
 		trusted = map (trustworthy . uuidnode) ts
-		uuidnode u = Dot.graphNode u $ M.findWithDefault "" u umap
+		uuidnode u = Dot.graphNode (show u) $ M.findWithDefault "" u umap
 
 hostname :: Git.Repo -> String
 hostname r
@@ -76,7 +76,7 @@ basehostname r = head $ split "." $ hostname r
  - or the remote name if not. -}
 repoName :: M.Map UUID String -> Git.Repo -> String
 repoName umap r
-	| null repouuid = fallback
+	| repouuid == NoUUID = fallback
 	| otherwise = M.findWithDefault fallback repouuid umap
 	where
 		repouuid = getUncachedUUID r
@@ -86,8 +86,8 @@ repoName umap r
 nodeId :: Git.Repo -> String
 nodeId r =
 	case getUncachedUUID r of
-		"" -> Git.repoLocation r
-		u -> u
+		NoUUID -> Git.repoLocation r
+		UUID u -> u
 
 {- A node representing a repo. -}
 node :: M.Map UUID String -> [Git.Repo] -> Git.Repo -> String