pre-commit: Update direct mode mappings.

Making the pre-commit hook look at git diff-index to find changed direct
mode files and update the mappings works pretty well.

One case where it does not work is when a file is git annex added, and then
git rmed, and then this is committed. That's a no-op commit, so the hook
probably doesn't even run, and it certianly never notices that the file
was deleted, so the mapping will still have the original filename in it.

For this and other reasons, it's important that the mappings still be
treated as possibly inconsistent.

Also, the assistant now allows the pre-commit hook to run when in direct
mode, so the mappings also get updated there.

Git/Ref.hs

@@ -37,6 +37,13 @@ exists :: Ref -> Repo -> IO Bool
 exists ref = runBool "show-ref" 
 	[Param "--verify", Param "-q", Param $ show ref]
 
+{- Checks if HEAD exists. It generally will, except for in a repository
+ - that was just created. -}
+headExists :: Repo -> IO Bool
+headExists repo = do
+	ls <- lines <$> pipeReadStrict [Param "show-ref", Param "--head"] repo
+	return $ any (" HEAD" `isSuffixOf`) ls
+
 {- Get the sha of a fully qualified git ref, if it exists. -}
 sha :: Branch -> Repo -> IO (Maybe Sha)
 sha branch repo = process <$> showref repo