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-fast-export.c

@@ -372,7 +372,7 @@ int cmd_fast_export(int argc, const char **argv, const char *prefix)
 	};
 
 	/* we handle encodings */
-	git_config(git_default_config);
+	git_config(git_default_config, NULL);
 
 	init_revisions(&revs, prefix);
 	argc = setup_revisions(argc, argv, &revs, NULL);