Make 'unpack_trees()' take the index to work on as an argument

This is just a very mechanical conversion, and makes everybody set it to
'&the_index' before calling, but at least it makes it more explicit
where we work with the index.

The next stage would be to split that index usage up into a 'source' and
a 'destination' index, so that we can unpack into a different index than
we started out from.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-commit.c

@@ -198,6 +198,7 @@ static void create_base_index(void)
 	opts.head_idx = 1;
 	opts.index_only = 1;
 	opts.merge = 1;
+	opts.index = &the_index;
 
 	opts.fn = oneway_merge;
 	tree = parse_tree_indirect(head_sha1);