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>

repository.h

@@ -1,7 +1,6 @@
 #ifndef REPOSITORY_H
 #define REPOSITORY_H
 
-#include "refs.h"
 #include "strmap.h"
 
 struct config_set;
@@ -27,6 +26,12 @@ enum fetch_negotiation_setting {
 	FETCH_NEGOTIATION_NOOP,
 };
 
+enum ref_storage_format {
+	REF_STORAGE_FORMAT_UNKNOWN,
+	REF_STORAGE_FORMAT_FILES,
+	REF_STORAGE_FORMAT_REFTABLE,
+};
+
 struct repo_settings {
 	int initialized;