add a small cache of the most recently accessed item from the git-annex branch

This will speed up typical cases like git-annex get, which currently
has to read the location log once, then read it a second time in order to
add a line to it. Since these reads now involve more than just reading
in a file, it seemed good to add a cache layer.

Only the most recent thing needs to be cached, because git-annex has
good locality; it operates on one file at a time, and only cares
about one item from the branch per file.

Types/Branch.hs

@@ -0,0 +1,16 @@
+{- git-annex branch data types
+ -
+ - Copyright 2011 Joey Hess <joey@kitenet.net>
+ -
+ - Licensed under the GNU GPL version 3 or higher.
+ -}
+
+module Types.Branch where
+
+data BranchCache = BranchCache {
+	cachedFile :: Maybe FilePath,
+	cachedContent :: String
+}
+
+emptyBranchCache :: BranchCache
+emptyBranchCache = BranchCache Nothing ""