balanced preferred content and --rebalance

This all works fine. But it doesn't check repository sizes yet, and
without repository size checking, once a repository gets full, there
will be no other repository that will want its files.

Use of sha2 seems unncessary, probably alder2 or md5 or crc would have
been enough. Possibly just summing up the bytes of the key mod the number
of repositories would have sufficed. But sha2 is there, and probably
hardware accellerated. I doubt very much there is any security benefit
to using it though. If someone wants to construct a key that will be
balanced onto a given repository, sha2 is certianly not going to stop
them.

Utility/Hash.hs

@@ -1,6 +1,6 @@
 {- Convenience wrapper around cryptonite's hashing.
  -
- - Copyright 2013-2021 Joey Hess <id@joeyh.name>
+ - Copyright 2013-2024 Joey Hess <id@joeyh.name>
  -
  - License: BSD-2-clause
  -}
@@ -16,6 +16,7 @@ module Utility.Hash (
 	sha2_224_context,
 	sha2_256,
 	sha2_256_context,
+	sha2_256s,
 	sha2_384,
 	sha2_384_context,
 	sha2_512,
@@ -106,6 +107,9 @@ sha2_256 = hashlazy
 sha2_256_context :: Context SHA256
 sha2_256_context = hashInit
 
+sha2_256s :: S.ByteString -> Digest SHA256
+sha2_256s = hash
+
 sha2_384 :: L.ByteString -> Digest SHA384
 sha2_384 = hashlazy