cache annex index filename for 1.5% speedup to queries

Types/IndexFiles.hs

@@ -0,0 +1,11 @@
+{- Alternative git index files
+ - 
+ - Copyright 2020 Joey Hess <id@joeyh.name>
+ -
+ - Licensed under the GNU AGPL version 3 or higher.
+ -}
+
+module Types.IndexFiles where
+
+data AltIndexFile = AnnexIndexFile | ViewIndexFile
+	deriving (Eq, Show)