remote rename: demonstrate a bogus "remote exists" bug

remote rename: demonstrate a bogus "remote exists" bug

Some users like to set `remote.origin.prune = true` in their ~/.gitconfig
so that all of their repositories use that default.

However, our code is ill-prepared for this, mistaking that single entry to
mean that there is already a remote of the name "origin", even if there is
not.

This patch adds a test case demonstrating this issue.

Reported by Andrew Arnott.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>