handle transitions with read-only unmerged git-annex branches

Capstone to this feature. Any transitions that have been performed on an
unmerged remote ref but not on the local git-annex branch, or vice-versa
have to be applied on the fly when reading files.

Sponsored-by: Dartmouth College's Datalad project

Types/Transitions.hs

@@ -0,0 +1,19 @@
+{- git-annex transitions data types
+ -
+ - Copyright 2021 Joey Hess <id@joeyh.name>
+ -
+ - Licensed under the GNU AGPL version 3 or higher.
+ -}
+
+module Types.Transitions where
+
+import Utility.RawFilePath
+
+import qualified Data.ByteString.Lazy as L
+import Data.ByteString.Builder
+
+data FileTransition
+	= ChangeFile Builder
+	| PreserveFile
+
+type TransitionCalculator = RawFilePath -> L.ByteString -> FileTransition