Make function to refill http queue a callback

This eliminates the last function provided by the code using http.h as
a global symbol, so it should be possible to have multiple programs
using http.h in the same executable, and it also adds an argument to
that callback, so that info can be passed into the callback without
being global.

Signed-off-by: Daniel Barkalow <barkalow@iabervon.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

http.h

@@ -70,10 +70,8 @@ extern void release_active_slot(struct active_request_slot *slot);
 
 #ifdef USE_CURL_MULTI
 extern void fill_active_slots(void);
+extern void add_fill_function(void *data, int (*fill)(void *));
 extern void step_active_slots(void);
-
-/* Provided by the program using http. */
-extern int fill_active_slot(void);
 #endif
 
 extern void http_init(void);