get_tree_entry: map blank requested entry to tree root

This means that
  git show HEAD:
will now return HEAD^{tree}, which is logically consistent with
  git show HEAD:Documentation
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <junkio@cox.net>

tree-walk.c

@@ -199,10 +199,17 @@ int get_tree_entry(const unsigned char *tree_sha1, const char *name, unsigned ch
 	int retval;
 	void *tree;
 	struct tree_desc t;
+	unsigned char root[20];
 
-	tree = read_object_with_reference(tree_sha1, tree_type, &t.size, NULL);
+	tree = read_object_with_reference(tree_sha1, tree_type, &t.size, root);
 	if (!tree)
 		return -1;
+
+	if (name[0] == '\0') {
+		hashcpy(sha1, root);
+		return 0;
+	}
+
 	t.buf = tree;
 	retval = find_tree_entry(&t, name, sha1, mode);
 	free(tree);