submodule: convert get_next_submodule to not rely on the_index

Instead of implicitly relying on the global 'the_index', convert
'get_next_submodule()' to use the index of the repository stored in the
callback data 'struct submodule_parallel_fetch'.

Since this removes the last user of the index compatibility macros,
define 'NO_THE_INDEX_COMPATIBILITY_MACROS' to prevent future users of
these macros in submodule.c.

Signed-off-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/fetch.c

@@ -3,6 +3,7 @@
  */
 #include "cache.h"
 #include "config.h"
+#include "repository.h"
 #include "refs.h"
 #include "commit.h"
 #include "builtin.h"
@@ -1397,7 +1398,8 @@ int cmd_fetch(int argc, const char **argv, const char *prefix)
 		struct argv_array options = ARGV_ARRAY_INIT;
 
 		add_options_to_argv(&options);
-		result = fetch_populated_submodules(&options,
+		result = fetch_populated_submodules(the_repository,
+						    &options,
 						    submodule_prefix,
 						    recurse_submodules,
 						    recurse_submodules_default,