more worktree improvements

Avoid more expensive code path when no core.worktree is configured.

Don't change worktree when reading config if one is already set.
This could happen if GIT_CORE_WORKTREE is set, and the repo also has
core.worktree, and the config is reread. Now GIT_CORE_WORKTREE will
prevail.

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 (worktree $ location r) Just <$> pathenv "GIT_WORK_TREE"
+	wt <- maybe (Git.Config.workTree r) Just <$> pathenv "GIT_WORK_TREE"
 	case wt of
 		Nothing -> return r
 		Just d -> do