refs: convert ref storage format to an enum

refs: convert ref storage format to an enum

The ref storage format is tracked as a simple unsigned integer, which
makes it harder than necessary to discover what that integer actually is
or where its values are defined.

Convert the ref storage format to instead be an enum.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>