Credential helpers are asked in turn until one of them give
positive response, which is cumbersome to turn off when you need to
run Git in an automated setting.  The credential helper interface
learned to allow a helper to say "stop, don't ask other helpers."
Also GIT_TERMINAL_PROMPT environment can be set to false to disable
our built-in prompt mechanism for passwords.

* jk/credential-quit:
  prompt: respect GIT_TERMINAL_PROMPT to disable terminal prompts
  credential: let helpers tell us to quit