avoid partial functions in Git.Url

After the last commit, it was able to throw errors just due to an
unparseable url. This avoids needing to worry about that, as long
as the call site has already checked that it has a parseable url.

Git/Construct.hs

@@ -121,16 +121,16 @@ localToUrl :: Repo -> Repo -> Repo
 localToUrl reference r
 	| not $ repoIsUrl reference = error "internal error; reference repo not url"
 	| repoIsUrl r = r
-	| otherwise = case Url.authority reference of
-		Nothing -> r
-		Just auth -> 
+	| otherwise = case (Url.authority reference, Url.scheme reference) of
+		(Just auth, Just s) -> 
 			let absurl = concat
-				[ Url.scheme reference
+				[ s
 				, "//"
 				, auth
 				, fromRawFilePath (repoPath r)
 				]
 			in r { location = Url $ fromJust $ parseURI absurl }
+		_ -> r
 
 {- Calculates a list of a repo's configured remotes, by parsing its config. -}
 fromRemotes :: Repo -> IO [Repo]