[BUG] remote-curl.c: honor pushurl

When doing a push (fetch, ..) over http(s), git calls git-remote-http to
communicate with the server.

	git-remote-http <remote> [<url>]

Git correctly honors a configured pushurl and passes it to git-remote-http,
but git-remote-http is initiating the http connection with the url defined
for remote (remote->url) rather than the passed url. This undermines the
purpose of a config like

	url = https://example.com/repo.git
	pushurl = https://user@example.com/repo.git

Introduced around 888692b7 - CC'ing Tay Ray Chuan. (I don't know if it was
working before, though.)