When adding a remote, we make sure that the remote does not exist
already. However, this test was not quite correct: when the
url.<...>.insteadOf config variable was set to the remote name to be
added, the code would assume that the remote exists already.

Let's allow adding remotes when there is a url.<...>.insteadOf setting
when both the name and the URL agree with the remote to be added.

It might seem like a mistake to compare against remote->url[0] without
verifying that remote->url_nr >=1, but at this point a missing URL has
been filled by the name already, therefore url_nr cannot be zero.

Noticed by Anastas Dancha.
Signed-off-by: NJohannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>