This code handles some special magic like *-deref and the
:short formatting specifier. The next patch will add another
field which outputs a ref and wants to use the same code.

This patch splits the "which ref are we outputting" from the
actual formatting. There should be no behavioral change.
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>