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.

Annex/AutoMerge.hs

@@ -42,8 +42,8 @@ import qualified Data.ByteString.Lazy as L
  - Callers should use Git.Branch.changed first, to make sure that
  - there are changes from the current branch to the branch being merged in.
  -}
-autoMergeFrom :: Git.Ref -> Maybe Git.Ref -> Bool -> Git.Branch.CommitMode -> Annex Bool
-autoMergeFrom branch currbranch inoverlay commitmode = do
+autoMergeFrom :: Git.Ref -> Maybe Git.Ref -> Git.Branch.CommitMode -> Annex Bool
+autoMergeFrom branch currbranch commitmode = do
 	showOutput
 	case currbranch of
 		Nothing -> go Nothing
@@ -52,7 +52,7 @@ autoMergeFrom branch currbranch inoverlay commitmode = do
 	go old = ifM isDirect
 		( mergeDirect currbranch old branch (resolveMerge old branch False) commitmode
 		, inRepo (Git.Merge.mergeNonInteractive branch commitmode)
-			<||> (resolveMerge old branch inoverlay <&&> commitResolvedMerge commitmode)
+			<||> (resolveMerge old branch False <&&> commitResolvedMerge commitmode)
 		)
 
 {- Resolves a conflicted merge. It's important that any conflicts be

Annex/Direct.hs

@@ -204,6 +204,7 @@ stageMerge d branch commitmode = do
 	-- has been updated, which would leave things in an inconsistent
 	-- state if mergeDirectCleanup is interrupted.
 	-- <http://marc.info/?l=git&m=140262402204212&w=2>
+	liftIO $ print ("stagemerge in", d)
 	merger <- ifM (coreSymlinks <$> Annex.getGitConfig)
 		( return Git.Merge.stageMerge
 		, return $ \ref -> Git.Merge.mergeNonInteractive ref commitmode

Command/Sync.hs

@@ -170,7 +170,7 @@ merge :: CurrBranch -> Git.Branch.CommitMode -> Git.Branch -> Annex Bool
 merge (Just b, Just adj) commitmode tomerge =
 	updateAdjustedBranch tomerge (b, adj) commitmode
 merge (b, _) commitmode tomerge =
-	autoMergeFrom tomerge b False commitmode
+	autoMergeFrom tomerge b commitmode
 
 syncBranch :: Git.Branch -> Git.Branch
 syncBranch = Git.Ref.under "refs/heads/synced" . fromDirectBranch . fromAdjustedBranch

Git/Merge.hs

@@ -15,12 +15,15 @@ import Git.Branch (CommitMode(..))
 
 {- Avoids recent git's interactive merge. -}
 mergeNonInteractive :: Ref -> CommitMode -> Repo -> IO Bool
-mergeNonInteractive branch commitmode
+mergeNonInteractive = mergeNonInteractive' []
+
+mergeNonInteractive' :: [CommandParam] -> Ref -> CommitMode -> Repo -> IO Bool
+mergeNonInteractive' extraparams branch commitmode
 	| older "1.7.7.6" = merge [Param $ fromRef branch]
 	| otherwise = merge $ [Param "--no-edit", Param $ fromRef branch]
   where
-	merge ps = runBool $ cp ++ [Param "merge"] ++ ps
-	cp
+	merge ps = runBool $ sp ++ [Param "merge"] ++ ps ++ extraparams
+	sp
 		| commitmode == AutomaticCommit =
 			[Param "-c", Param "commit.gpgsign=false"]
 		| otherwise = []