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

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>