encoding a git sha as a git-annex key

This is a bijective mapping, and is distinct from SHA1.

As git transitions away from sha1, this could contain whatever hash
git uses.

Key.hs

@@ -5,6 +5,7 @@
  - Licensed under the GNU AGPL version 3 or higher.
  -}
 
+{-# LANGUAGE OverloadedStrings #-}
 {-# OPTIONS_GHC -fno-warn-orphans #-}
 
 module Key (
@@ -22,6 +23,8 @@ module Key (
 	nonChunkKey,
 	chunkKeyOffset,
 	isChunkKey,
+	gitShaKey,
+	keyGitSha,
 	isKeyPrefix,
 	splitKeyNameExtension,
 
@@ -35,6 +38,7 @@ import qualified Data.Attoparsec.ByteString as A
 
 import Common
 import Types.Key
+import Git.Types
 import Utility.QuickCheck
 import Utility.Bloom
 import Utility.Aeson
@@ -58,6 +62,23 @@ chunkKeyOffset k = (*)
 isChunkKey :: Key -> Bool
 isChunkKey k = isJust (fromKey keyChunkSize k) && isJust (fromKey keyChunkNum k)
 
+-- Encodes a git sha as a key.
+--
+-- This is not the same as a SHA1 key, because the mapping needs to be
+-- bijective, also because git may not always use SHA1.
+gitShaKey :: Sha -> Key
+gitShaKey (Ref s) = mkKey $ \kd -> kd
+	{ keyName = s
+	, keyVariety = OtherKey "GIT"
+	}
+
+-- Reverse of gitShaKey
+keyGitSha :: Key -> Maybe Sha
+keyGitSha k
+	| fromKey keyVariety k == OtherKey "GIT" =
+		Just (Ref (fromKey keyName k))
+	| otherwise = Nothing
+
 serializeKey :: Key -> String
 serializeKey = decodeBS' . serializeKey'