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/Threads/Merger.hs

@@ -12,7 +12,6 @@ import Assistant.TransferQueue
 import Assistant.BranchChange
 import Assistant.DaemonStatus
 import Assistant.ScanRemotes
-import Assistant.Sync
 import Utility.DirWatcher
 import Utility.DirWatcher.Types
 import qualified Annex.Branch
@@ -86,7 +85,7 @@ onChange file
 					, "into", Git.fromRef b
 					]
 				void $ liftAnnex $ Command.Sync.merge
-					currbranch mergeConfig
+					currbranch Command.Sync.mergeConfig
 					Git.Branch.AutomaticCommit
 					changedbranch
 	mergecurrent _ = noop