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>

pager.c

@@ -33,7 +33,7 @@ void setup_pager(void)
 		return;
 	if (!pager) {
 		if (!pager_program)
-			git_config(git_default_config);
+			git_config(git_default_config, NULL);
 		pager = pager_program;
 	}
 	if (!pager)