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>
diff --git a/builtin-mv.c b/builtin-mv.c
index 94f6dd2..3edebef 100644
--- a/builtin-mv.c
+++ b/builtin-mv.c
@@ -81,7 +81,7 @@
 	struct path_list deleted = {NULL, 0, 0, 0};
 	struct path_list changed = {NULL, 0, 0, 0};
 
-	git_config(git_default_config);
+	git_config(git_default_config, NULL);
 
 	newfd = hold_locked_index(&lock_file, 1);
 	if (read_cache() < 0)