credential: add a field called "ephemeral"

Now that we have support for a wide variety of types of authentication, it's important to indicate to other credential helpers whether they should store credentials, since not every credential helper may intuitively understand all possible values of the authtype field. Do so with a boolean field called "ephemeral", to indicate whether the credential is expected to be temporary. For example, in HTTP Digest authentication, the Authorization header value is based off a nonce. It isn't useful to store this value for later use because reusing the credential long term will not result in successful authentication due to the nonce necessarily differing. An additional case is potentially short-lived credentials, which may last only a few hours. It similarly wouldn't be helper for other credential helpers to attempt to provide these much later. We do still pass the value to "git credential store" or "git credential erase", since it may be helpful to the original helper to know whether the operation was successful. Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>