Bugfix: Direct mode no longer repeatedly checksums duplicated files.

Fixed by storing a list of cached inodes for a key, instead of just one.

Backwards compatability note: An old git-annex version will fail to parse
an inode cache file that has been written by a new version, and has
multiple items. It will succees if just one. So old git-annexes will have
even worse behavior when there are duplicated files, if that is possible.
I don't think it will be a problem. (Famous last words.)

Also, note that it doesn't expire old and unused inode caches for a key.
It would be possible to add this if needed; just look through the
associated files for a key and if there are more cached inodes, throw out
any not corresponding to associated files. Unless a file is being copied
repeatedly and the old copy deleted, this lack of expiry should not be a
problem.

Command/Add.hs

@@ -132,7 +132,7 @@ ingest (Just source) = do
 	goindirect Nothing _ = failure
 
 	godirect (Just (key, _)) (Just cache) = do
-		writeInodeCache key cache
+		addInodeCache key cache
 		finishIngestDirect key source
 		return $ Just key
 	godirect _ _ = failure