better directory handling

Rename Locations functions for better consitency, and make their values
more consistent too.

Used </> rather than manually building paths. There are still more places
that manually do so, but are tricky, due to the behavior of </> when
the second FilePath is absolute. So I only changed places where
it obviously was relative.

Backend/SHA1.hs

@@ -48,7 +48,7 @@ keyValue file = do
 checkKeySHA1 :: Key -> Annex Bool
 checkKeySHA1 key = do
 	g <- Annex.gitRepo
-	let file = annexLocation g key
+	let file = gitAnnexLocation g key
 	present <- liftIO $ doesFileExist file
 	if not present
 		then return True