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-merge-base.c

@@ -28,7 +28,7 @@ int cmd_merge_base(int argc, const char **argv, const char *prefix)
 	unsigned char rev1key[20], rev2key[20];
 	int show_all = 0;
 
-	git_config(git_default_config);
+	git_config(git_default_config, NULL);
 
 	while (1 < argc && argv[1][0] == '-') {
 		const char *arg = argv[1];