optimize index updating

The last branch ref that the index was updated to is stored in
.git/annex/index.lck, and the index only updated when the current
branch ref differs.

(The .lck file should later be used for locking too.)

Some more optimization is still needed, since there is some redundancy in
calls to git show-ref.

Git.hs

@@ -463,8 +463,8 @@ shaSize :: Int
 shaSize = 40
 
 {- Commits the index into the specified branch (or other ref), 
- - with the specified parent refs. -}
-commit :: String -> Ref -> [Ref] -> Repo -> IO ()
+ - with the specified parent refs, and returns the new ref -}
+commit :: String -> Ref -> [Ref] -> Repo -> IO Ref
 commit message newref parentrefs repo = do
 	tree <- getSha "write-tree" $ asString $
 		pipeRead [Param "write-tree"] repo
@@ -473,6 +473,7 @@ commit message newref parentrefs repo = do
 			(map Param $ ["commit-tree", show tree] ++ ps)
 			(L.pack message) repo
 	run "update-ref" [Param $ show newref, Param $ show sha] repo
+	return sha
 	where
 		ignorehandle a = snd <$> a
 		asString a = L.unpack <$> a