gitcredentials(7): make shell-snippet example more realistic

There's an example of using your own bit of shell to act as a credential helper, but it's not very realistic: - It's stupid to hand out your secret password to _every_ host. In the real world you'd use the config-matcher to limit it to a particular host. - We never provided a username. We can easily do that in another config option (you can do it in the helper, too, but this is much more readable). - We were sending the secret even for store/erase operations. This is OK because Git would just ignore it, but a real system would probably be unlocking a password store, which you wouldn't want to do more than necessary. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>