builtin: add a repository parameter for builtin functions

In order to reduce the usage of the global the_repository, add a
parameter to builtin functions that will get passed a repository
variable.

This commit uses UNUSED on most of the builtin functions, as subsequent
commits will modify the actual builtins to pass the repository parameter
down.

Signed-off-by: John Cai <johncai86@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/diff-index.c

@@ -14,7 +14,10 @@ static const char diff_cache_usage[] =
 "\n"
 COMMON_DIFF_OPTIONS_HELP;
 
-int cmd_diff_index(int argc, const char **argv, const char *prefix)
+int cmd_diff_index(int argc,
+		   const char **argv,
+		   const char *prefix,
+		   struct repository *repo UNUSED)
 {
 	struct rev_info rev;
 	unsigned int option = 0;