http-push.c: use a faux remote to pass to http_init

http-push.c: use a faux remote to pass to http_init

This patch allows http_push to use http authentication via prompts.
You may notice that there is a remote struct that only contains the
url from the repo struct.  This struct is a temporary fix for a larger
issue, but gets http authentication via prompts out the door, and
keeps users from having to store passwords in plain text files.

Signed-off-by: Amos King <amos.l.king@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>