Sped up git-annex init in a clone of an existing repository

Seems that hasOrigin was never finding origin's git-annex branch, so a new
one got created each time. And so then it later needed to merge the two
branches, which is expensive.

Added --no-track to git branch to avoid it displaying a message about
setting up tracking branches. Of course there's no reason to make the
git-annex branch a tracking branch since git-annex auto-merges it.

Git/Ref.hs

@@ -84,7 +84,8 @@ dateRef r (RefDate d) = Ref $ fromRef' r <> "@" <> encodeBS' d
 fileFromRef :: Ref -> RawFilePath -> Ref
 fileFromRef r f = let (Ref fr) = fileRef f in Ref (fromRef' r <> fr)
 
-{- Checks if a ref exists. -}
+{- Checks if a ref exists. Note that it must be fully qualified,
+ - eg refs/heads/master rather than master. -}
 exists :: Ref -> Repo -> IO Bool
 exists ref = runBool
 	[ Param "show-ref"