refactor git-annex branch log filename code into central location

Having one module that knows about all the filenames used on the branch
allows working back from an arbitrary filename to enough information about
it to implement dropping dead remotes and doing other log file compacting
as part of a forget transition.

Logs/PreferredContent.hs

@@ -24,6 +24,7 @@ import Data.Time.Clock.POSIX
 import Common.Annex
 import qualified Annex.Branch
 import qualified Annex
+import Logs
 import Logs.UUIDBased
 import Limit
 import qualified Utility.Matcher
@@ -35,10 +36,6 @@ import Logs.Group
 import Logs.Remote
 import Types.StandardGroups
 
-{- Filename of preferred-content.log. -}
-preferredContentLog :: FilePath
-preferredContentLog = "preferred-content.log"
-
 {- Changes the preferred content configuration of a remote. -}
 preferredContentSet :: UUID -> String -> Annex ()
 preferredContentSet uuid@(UUID _) val = do