Patch for http-fetch.c and older curl releases

Patch for http-fetch.c and older curl releases

Older curl releases do not define CURLE_HTTP_RETURNED_ERROR, they
use CURLE_HTTP_NOT_FOUND instead. Newer curl releases keep the
CURLE_HTTP_NOT_FOUND definition but using a -DCURL_NO_OLDIES
preprocessor flag the old name will not be present in the 'curl.h'
header.

This patch makes our code written for newer releases of the curl
library but allow compiling against an older curl (older than
0x070a03) by defining the missing CURLE_HTTP_RETURNED_ERROR as a
synonym for CURLE_HTTP_NOT_FOUND.

Signed-off-by: Art Haas <ahaas@airmail.net>
Signed-off-by: Junio C Hamano <junkio@cox.net>