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.

git-union-merge.hs

@@ -42,7 +42,7 @@ main :: IO ()
 main = do
 	[aref, bref, newref] <- parseArgs
 	g <- Git.configRead =<< Git.repoFromCwd
-	Git.useIndex (tmpIndex g)
+	_ <- Git.useIndex (tmpIndex g)
 	setup g
 	GitUnionMerge.merge g [aref, bref]
 	GitUnionMerge.commit g "union merge" newref [aref, bref]