Make function to refill http queue a callback

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>