reftable/system: stop depending on "hash.h"

We include "hash.h" in "reftable/system.h" such that we can use hash
format IDs as well as the raw size of SHA1 and SHA256. As we are in the
process of converting the reftable library to become standalone we of
course cannot rely on those constants anymore.

Introduce a new `enum reftable_hash` to replace internal uses of the
hash format IDs and new constants that replace internal uses of the hash
size. Adapt the reftable backend to set up the correct hash function.

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

reftable/reftable-writer.h

@@ -33,7 +33,7 @@ struct reftable_write_options {
 	/* 4-byte identifier ("sha1", "s256") of the hash.
 	 * Defaults to SHA1 if unset
 	 */
-	uint32_t hash_id;
+	enum reftable_hash hash_id;
 
 	/* Default mode for creating files. If unset, use 0666 (+umask) */
 	unsigned int default_permissions;