add journaling to speed up changes to the git-annex branch

git is slow when the index file is large and has to be rewritten each time
a file is changed. To speed this up, added a journal where changes are
recorded before being fed into the index file and committed to the
git-annex branch. The entire journal can be fed into git with just 2
commands, and only one write of the index file.

Types/BranchState.hs

@@ -9,10 +9,9 @@ module Types.BranchState where
 
 data BranchState = BranchState {
 	branchUpdated :: Bool,
-	branchChanged :: Bool,
 	cachedFile :: Maybe FilePath,
 	cachedContent :: String
 }
 
 startBranchState :: BranchState
-startBranchState = BranchState False False Nothing ""
+startBranchState = BranchState False Nothing ""