config: mark unused callback parameters

The callback passed to git_config() must conform to a particular
interface. But most callbacks don't actually look at the extra "void
*data" parameter. Let's mark the unused parameters to make
-Wunused-parameter happy.

Note there's one unusual case here in get_remote_default() where we
actually ignore the "value" parameter. That's because it's only checking
whether the option is found at all, and not parsing its value.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

color.c

@@ -415,7 +415,7 @@ int want_color_fd(int fd, int var)
 	return var;
 }
 
-int git_color_config(const char *var, const char *value, void *cb)
+int git_color_config(const char *var, const char *value, void *UNUSED(cb))
 {
 	if (!strcmp(var, "color.ui")) {
 		git_use_color_default = git_config_colorbool(var, value);