refs: avoid include cycle with "repository.h"

refs: avoid include cycle with "repository.h"

There is an include cycle between "refs.h" and "repository.h" via
"commit.h", "object.h" and "hash.h". This has the effect that several
definitions of structs and enums will not be visible once we merge
"hash-ll.h" back into "hash.h" in the next commit.

The only reason that "repository.h" includes "refs.h" is the definition
of `enum ref_storage_format`. Move it into "repository.h" and have
"refs.h" include "repository.h" instead to fix the cycle.

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