http.c: Use curl_multi_fdset to select on curl fds instead of just sleeping

http.c: Use curl_multi_fdset to select on curl fds instead of just sleeping

Instead of sleeping unconditionally for a 50ms, when no data can be read
from the http connection(s), use curl_multi_fdset() to obtain the actual
file descriptors of the open connections and use them in the select call.
This way, the 50ms sleep is interrupted when new data arrives.

Signed-off-by: Mika Fischer <mika.fischer@zoopnet.de>
Helped-by: Daniel Stenberg <daniel@haxx.se>
Signed-off-by: Junio C Hamano <gitster@pobox.com>