Provide git_config with a callback-data parameter

git_config() only had a function parameter, but no callback data
parameter.  This assumes that all callback functions only modify
global variables.

With this patch, every callback gets a void * parameter, and it is hoped
that this will help the libification effort.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-diff-index.c

@@ -17,7 +17,7 @@ int cmd_diff_index(int argc, const char **argv, const char *prefix)
 	int result;
 
 	init_revisions(&rev, prefix);
-	git_config(git_diff_basic_config); /* no "diff" UI options */
+	git_config(git_diff_basic_config, NULL); /* no "diff" UI options */
 	rev.abbrev = 0;
 
 	argc = setup_revisions(argc, argv, &rev, NULL);