add ChunkMethod type and make Logs.Chunk use it, rather than assuming fixed size chunks (so eg, rolling hash chunks can be supported later)

If a newer git-annex starts logging something else in the chunk log, it
won't be used by this version, but it will be preserved when updating the
log.

Logs/Chunk/Pure.hs

@@ -6,7 +6,8 @@
  -}
 
 module Logs.Chunk.Pure 
-	( ChunkSize
+	( ChunkMethod(..)
+	, ChunkSize
 	, ChunkCount
 	, ChunkLog
 	, parseLog
@@ -17,24 +18,37 @@ import Common.Annex
 import Logs.MapLog
 import Data.Int
 
+-- Currently chunks are all fixed size, but other chunking methods
+-- may be added.
+data ChunkMethod = FixedSizeChunks ChunkSize | UnknownChunks String
+	deriving (Ord, Eq)
+
 type ChunkSize = Int64
 
+-- 0 when chunks are no longer present
 type ChunkCount = Integer
 
-type ChunkLog = MapLog (UUID, ChunkSize) ChunkCount
+type ChunkLog = MapLog (UUID, ChunkMethod) ChunkCount
+
+parseChunkMethod :: String -> ChunkMethod
+parseChunkMethod s = maybe (UnknownChunks s) FixedSizeChunks (readish s)
+
+showChunkMethod :: ChunkMethod -> String
+showChunkMethod (FixedSizeChunks sz) = show sz
+showChunkMethod (UnknownChunks s) = s
 
 parseLog :: String -> ChunkLog
 parseLog = parseMapLog fieldparser valueparser
   where
 	fieldparser s =
-		let (u,sz) = separate (== sep) s
-		in (,) <$> pure (toUUID u) <*> readish sz
+		let (u,m) = separate (== sep) s
+		in Just (toUUID u, parseChunkMethod m)
 	valueparser = readish
 
 showLog :: ChunkLog -> String
 showLog = showMapLog fieldshower valueshower
   where
-	fieldshower (u, sz) = fromUUID u ++ sep : show sz
+	fieldshower (u, m) = fromUUID u ++ sep : showChunkMethod m
 	valueshower = show
 
 sep :: Char