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/File.hs

@@ -77,7 +77,7 @@ copyKeyFile key file = do
 		-- before going on to the next remote.)
 		probablyPresent r =
 			if not $ Git.repoIsUrl r
-				then liftIO $ doesFileExist $ annexLocation r key
+				then liftIO $ doesFileExist $ gitAnnexLocation r key
 				else return True
 		docopy r continue = do
 			showNote $ "copying from " ++ Git.repoDescribe r ++ "..."