A relative core.worktree is relative to the gitdir.

Now that this is handled correctly, git-annex can be used in git submodules.

Also, fixed infelicity where Git.CurrentRepo and Git.Config.updateLocation
were both dealing with core.worktree. Now updateLocation handles it for
Local as well as for LocalUnknown repos.

Git/CurrentRepo.hs

@@ -31,7 +31,7 @@ get :: IO Repo
 get = do
 	gd <- pathenv "GIT_DIR"
 	r <- configure gd =<< maybe fromCwd fromPath gd
-	wt <- maybe (Git.Config.workTree r) Just <$> pathenv "GIT_WORK_TREE"
+	wt <- maybe (worktree $ location r) Just <$> pathenv "GIT_WORK_TREE"
 	case wt of
 		Nothing -> return r
 		Just d -> do