Sped back up fsck, copy --from etc

All commands that often have to read a lot of information from
the git-annex branch should now be nearly as fast as before
the branch was introduced.

Before fsck was taking approximatly 3 hours, now it's running in 8 minutes.

The code is very nasty. It should be rewritten to read the header line
from git cat-file, and then read the specified number of bytes of content.

Types/BranchState.hs

@@ -7,11 +7,18 @@
 
 module Types.BranchState where
 
+import System.IO
+
 data BranchState = BranchState {
-	branchUpdated :: Bool,
+	branchUpdated :: Bool, -- has the branch been updated this run?
+
+	-- (from, to) handles used to talk to a git-cat-file process
+	catFileHandles :: Maybe (Handle, Handle),
+
+	-- the content of one file is cached
 	cachedFile :: Maybe FilePath,
 	cachedContent :: String
 }
 
 startBranchState :: BranchState
-startBranchState = BranchState False Nothing ""
+startBranchState = BranchState False Nothing Nothing ""