credential: let helpers tell us to quit

When we are trying to fill a credential, we loop over the
set of defined credential-helpers, then fall back to running
askpass, and then finally prompt on the terminal. Helpers
which cannot find a credential are free to tell us nothing,
but they cannot currently ask us to stop prompting.

This patch lets them provide a "quit" attribute, which asks
us to stop the process entirely (avoiding running more
helpers, as well as the askpass/terminal prompt).

This has a few possible uses:

  1. A helper which prompts the user itself (e.g., in a
     dialog) can provide a "cancel" button to the user to
     stop further prompts.

  2. Some helpers may know that prompting cannot possibly
     work. For example, if their role is to broker a ticket
     from an external auth system and that auth system
     cannot be contacted, there is no point in continuing
     (we need a ticket to authenticate, and the user cannot
     provide one by typing it in).

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

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