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-apply.c b/builtin-apply.c
index 1103625..bbdf08a 100644
--- a/builtin-apply.c
+++ b/builtin-apply.c
@@ -2979,11 +2979,11 @@
 	return 0;
 }
 
-static int git_apply_config(const char *var, const char *value)
+static int git_apply_config(const char *var, const char *value, void *cb)
 {
 	if (!strcmp(var, "apply.whitespace"))
 		return git_config_string(&apply_default_whitespace, var, value);
-	return git_default_config(var, value);
+	return git_default_config(var, value, cb);
 }
 
 
@@ -2999,7 +2999,7 @@
 
 	prefix = setup_git_directory_gently(&is_not_gitdir);
 	prefix_length = prefix ? strlen(prefix) : 0;
-	git_config(git_apply_config);
+	git_config(git_apply_config, NULL);
 	if (apply_default_whitespace)
 		parse_whitespace_option(apply_default_whitespace);