http_init: accept separate URL parameter

http_init: accept separate URL parameter

The http_init function takes a "struct remote". Part of its
initialization procedure is to look at the remote's url and
grab some auth-related parameters. However, using the url
included in the remote is:

  - wrong; the remote-curl helper may have a separate,
    unrelated URL (e.g., from remote.*.pushurl). Looking at
    the remote's configured url is incorrect.

  - incomplete; http-fetch doesn't have a remote, so passes
    NULL. So http_init never gets to see the URL we are
    actually going to use.

  - cumbersome; http-push has a similar problem to
    http-fetch, but actually builds a fake remote just to
    pass in the URL.

Instead, let's just add a separate URL parameter to
http_init, and all three callsites can pass in the
appropriate information.

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