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-http-fetch.c b/builtin-http-fetch.c
index b1f3389..3a06248 100644
--- a/builtin-http-fetch.c
+++ b/builtin-http-fetch.c
@@ -18,7 +18,7 @@
 	int get_verbosely = 0;
 	int get_recover = 0;
 
-	git_config(git_default_config);
+	git_config(git_default_config, NULL);
 
 	while (arg < argc && argv[arg][0] == '-') {
 		if (argv[arg][1] == 't') {