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-rev-parse.c

@@ -381,7 +381,7 @@ int cmd_rev_parse(int argc, const char **argv, const char *prefix)
 		return cmd_parseopt(argc - 1, argv + 1, prefix);
 
 	prefix = setup_git_directory();
-	git_config(git_default_config);
+	git_config(git_default_config, NULL);
 	for (i = 1; i < argc; i++) {
 		const char *arg = argv[i];