run out of tree merge with --no-ff

This is how direct mode does it too, and somehow, for reasons that
currently escape me, this makes git merge not care if it's run with an
empty work tree.

Annex/AdjustedBranch.hs

@@ -32,6 +32,7 @@ import qualified Git.Ref
 import qualified Git.Command
 import qualified Git.Tree
 import qualified Git.DiffTree
+import qualified Git.Merge
 import Git.Tree (TreeItem(..))
 import Git.Sha
 import Git.Env
@@ -272,13 +273,16 @@ updateAdjustedBranch tomerge (origbranch, adj) commitmode = catchBoolIO $
 			withemptydir tmpwt $ withWorkTree tmpwt $ do
 				liftIO $ writeFile (tmpgit </> "HEAD") (fromRef updatedorig)
 				showAction $ "Merging into " ++ fromRef (Git.Ref.base origbranch)
-				ifM (autoMergeFrom tomerge (Just origbranch) True commitmode)
-					( do
+				-- The --no-ff is important; it makes git
+				-- merge not care that the work tree is empty.
+				merged <- inRepo (Git.Merge.mergeNonInteractive' [Param "--no-ff"] tomerge commitmode)
+					<||> (resolveMerge (Just updatedorig) tomerge True <&&> commitResolvedMerge commitmode)
+				if merged
+					then do
 						!mergecommit <- liftIO $ extractSha <$> readFile (tmpgit </> "HEAD")
 						-- This is run after the commit lock is dropped.
 						return $ postmerge currbranch mergecommit
-					, return $ return False
-					)
+					else return $ return False
 	changestomerge Nothing _ = return $ return False
 	
 	withemptydir d a = bracketIO setup cleanup (const a)
@@ -305,7 +309,7 @@ updateAdjustedBranch tomerge (origbranch, adj) commitmode = catchBoolIO $
 		adjmergecommit <- commitAdjustedTree' adjtree mergecommit
 			[mergecommit, currbranch]
 		showAction "Merging into adjusted branch"
-		ifM (autoMergeFrom adjmergecommit (Just currbranch) False commitmode)
+		ifM (autoMergeFrom adjmergecommit (Just currbranch) commitmode)
 			-- The adjusted branch has a merge commit on top;
 			-- clean that up and propigate any changes made
 			-- in that merge to the origbranch.