checkout: work from a subdirectory

When switching branches from a subdirectory, checkout rewritten
in C extracted the toplevel of the tree in there.

This should fix it.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-checkout.c

@@ -237,7 +237,6 @@ static int merge_working_tree(struct checkout_opts *opts,
 			topts.dir = xcalloc(1, sizeof(*topts.dir));
 			topts.dir->show_ignored = 1;
 			topts.dir->exclude_per_dir = ".gitignore";
-			topts.prefix = prefix;
 			tree = parse_tree_indirect(old->commit->object.sha1);
 			init_tree_desc(&trees[0], tree->buffer, tree->size);
 			tree = parse_tree_indirect(new->commit->object.sha1);

t/t7201-co.sh

@@ -263,4 +263,38 @@ test_expect_success 'checkout with ambiguous tag/branch names' '
 
 '
 
+test_expect_success 'switch branches while in subdirectory' '
+
+	git reset --hard &&
+	git checkout master &&
+
+	mkdir subs &&
+	(
+		cd subs &&
+		git checkout side
+	) &&
+	! test -f subs/one &&
+	rm -fr subs
+
+'
+
+test_expect_success 'checkout specific path while in subdirectory' '
+
+	git reset --hard &&
+	git checkout side &&
+	mkdir subs &&
+	>subs/bero &&
+	git add subs/bero &&
+	git commit -m "add subs/bero" &&
+
+	git checkout master &&
+	mkdir -p subs &&
+	(
+		cd subs &&
+		git checkout side -- bero
+	) &&
+	test -f subs/bero
+
+'
+
 test_done