builtin: stop using `the_index`

Convert builtins to use `the_repository->index` instead of `the_index`.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/builtin/diff-tree.c b/builtin/diff-tree.c
index a8e68ce..0d3c611 100644
--- a/builtin/diff-tree.c
+++ b/builtin/diff-tree.c
@@ -1,4 +1,3 @@
-#define USE_THE_INDEX_VARIABLE
 #include "builtin.h"
 #include "config.h"
 #include "diff.h"
@@ -206,7 +205,7 @@ int cmd_diff_tree(int argc, const char **argv, const char *prefix)
 		opt->diffopt.rotate_to_strict = 0;
 		opt->diffopt.no_free = 1;
 		if (opt->diffopt.detect_rename) {
-			if (!the_index.cache)
+			if (the_repository->index->cache)
 				repo_read_index(the_repository);
 			opt->diffopt.setup |= DIFF_SETUP_USE_SIZE_CACHE;
 		}