refactor RemoteTrackingBranch

Not specific to Import; export will use it too.

Annex/RemoteTrackingBranch.hs

@@ -0,0 +1,34 @@
+{- git-annex remote tracking branches
+ -
+ - Copyright 2019 Joey Hess <id@joeyh.name>
+ -
+ - Licensed under the GNU AGPL version 3 or higher.
+ -}
+
+module Annex.RemoteTrackingBranch
+	( RemoteTrackingBranch
+	, mkRemoteTrackingBranch
+	, fromRemoteTrackingBranch
+	, setRemoteTrackingBranch
+	) where
+
+import Annex.Common
+import Git.Types
+import qualified Git.Ref
+import qualified Git.Branch
+import qualified Types.Remote as Remote
+
+newtype RemoteTrackingBranch = RemoteTrackingBranch
+	{ fromRemoteTrackingBranch :: Ref }
+	deriving (Show, Eq)
+
+{- Makes a remote tracking branch corresponding to a local branch. 
+ - Note that the local branch does not need to exist yet. -}
+mkRemoteTrackingBranch :: Remote -> Branch -> RemoteTrackingBranch
+mkRemoteTrackingBranch remote ref = RemoteTrackingBranch $
+	Git.Ref.underBase ("refs/remotes/" ++ Remote.name remote) ref
+
+{- Set remote tracking branch to point to a commit. -}
+setRemoteTrackingBranch :: RemoteTrackingBranch -> Sha -> Annex ()
+setRemoteTrackingBranch tb commit = 
+	inRepo $ Git.Branch.update' (fromRemoteTrackingBranch tb) commit