http.c: Use timeout suggested by curl instead of fixed 50ms timeout

http.c: Use timeout suggested by curl instead of fixed 50ms timeout

Recent versions of curl can suggest a period of time the library user
should sleep and try again, when curl is blocked on reading or writing
(or connecting). Use this timeout instead of always sleeping for 50ms.

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>