http: retry authentication failures for all http requests

http: retry authentication failures for all http requests

Commit 42653c0 (Prompt for a username when an HTTP request
401s, 2010-04-01) changed http_get_strbuf to prompt for
credentials when we receive a 401, but didn't touch
http_get_file. The latter is called only for dumb http;
while it's usually the case that people don't use
authentication on top of dumb http, there is no reason not
to allow both types of requests to use this feature.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>