remote: create fetch config lines with '+'

remote: create fetch config lines with '+'

Since git-remote always uses remote tracking branches, it
should be safe to always force updates of those branches.
I.e., we should generate

  fetch = +refs/heads/*:refs/remotes/$remote/*

instead of

  fetch = refs/heads/*:refs/remotes/$remote/*

This was the behavior of the perl version, which seems to
have been lost in the C rewrite.

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