Avoid using git commit in direct mode, since in some situations it will read the full contents of files in the tree.

The assistant's commit code also always avoids git commit, for simplicity.
Indirect mode sync still does a git commit -a to catch unstaged changes.

Note that this means that direct mode sync no longer runs the pre-commit
hook or any other hooks git commit might call. The git annex pre-commit
hook action for direct mode is however explicitly run. (The assistant
already ran git commit with hooks disabled, so no change there.)

Git/Ref.hs

@@ -10,6 +10,7 @@ module Git.Ref where
 import Common
 import Git
 import Git.Command
+import Git.Sha
 
 import Data.Char (chr)
 
@@ -105,6 +106,11 @@ matchingUniq refs repo = nubBy uniqref <$> matching refs repo
   where
 	uniqref (a, _) (b, _) = a == b
 
+{- Gets the sha of the tree a ref uses. -}
+tree :: Ref -> Repo -> IO (Maybe Sha)
+tree ref = extractSha <$$> pipeReadStrict
+	[ Param "rev-parse", Param (show ref ++ ":") ]
+
 {- Checks if a String is a legal git ref name.
  -
  - The rules for this are complex; see git-check-ref-format(1) -}