clean up struct ref's nonfastforward field

Each ref structure contains a "nonfastforward" field which is set during push to show whether the ref rewound history. Originally this was a single bit, but it was changed in f25950f (push: Provide situational hints for non-fast-forward errors) to an enum differentiating a non-ff of the current branch versus another branch. However, we never actually set the member according to the enum values, nor did we ever read it expecting anything but a boolean value. But we did use the side effect of declaring the enum constants to store those values in a totally different integer variable. The code as-is isn't buggy, but the enum declaration inside "struct ref" is somewhat misleading. Let's convert nonfastforward back into a single bit, and then define the NON_FF_* constants closer to where they would be used (they are returned via the "int *nonfastforward" parameter to transport_push, so we can define them there). Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>