Add support for core.worktree, and fix support for GIT_WORK_TREE and GIT_DIR.

The environment needs to override git-config. Changed when git config is
read, and avoid rereading it once it's been read.

chdir for both worktree settings.

Remote/Git.hs

@@ -179,12 +179,8 @@ repoAvail r
  - monad using that repository. -}
 onLocal :: Git.Repo -> Annex a -> IO a
 onLocal r a = do
-	-- Avoid re-reading the repository's configuration if it was
-	-- already read.
-	state <- if M.null $ Git.config r
-		then Annex.new r
-		else return $ Annex.newState r
-	Annex.eval state $ do
+	s <- Annex.new r
+	Annex.eval s $ do
 		-- No need to update the branch; its data is not used
 		-- for anything onLocal is used to do.
 		Annex.BranchState.disableUpdate