sync: Pass --allow-unrelated-histories to git merge when used with git git 2.9.0 or newer.

This makes merging a remote into a freshly created direct mode repository
work the same as it works in indirect mode.

The git-annex branches would get merged in any case by a sync,
since that doesn't use git merge.

This might need to be revisited later to better mirror git's behavior.

Assistant/Sync.hs

@@ -21,7 +21,6 @@ import Utility.Parallel
 import qualified Git
 import qualified Git.Command
 import qualified Git.Ref
-import qualified Git.Merge
 import qualified Remote
 import qualified Types.Remote as Remote
 import qualified Remote.List as Remote
@@ -239,19 +238,12 @@ manualPull currentbranch remotes = do
 			)
 	haddiverged <- liftAnnex Annex.Branch.forceUpdate
 	forM_ normalremotes $ \r ->
-		liftAnnex $ Command.Sync.mergeRemote r currentbranch mergeConfig
+		liftAnnex $ Command.Sync.mergeRemote r currentbranch Command.Sync.mergeConfig
 	u <- liftAnnex getUUID
 	forM_ xmppremotes $ \r ->
 		sendNetMessage $ Pushing (getXMPPClientID r) (PushRequest u)
 	return (catMaybes failed, haddiverged)
 
-mergeConfig :: [Git.Merge.MergeConfig]
-mergeConfig = 
-	[ Git.Merge.MergeNonInteractive
-	-- Pairing involves merging unrelated histories
-	, Git.Merge.MergeUnrelatedHistories
-	]
-
 {- Start syncing a remote, using a background thread. -}
 syncRemote :: Remote -> Assistant ()
 syncRemote remote = do