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.

Command/SendKey.hs

@@ -32,7 +32,7 @@ start keyname = do
 	let key = genKey (head backends) keyname
 	present <- inAnnex key
 	g <- Annex.gitRepo
-	let file = annexLocation g key
+	let file = gitAnnexLocation g key
 	when present $
 		liftIO $ rsyncServerSend file
 	liftIO exitFailure