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/Remote.hs

@@ -25,12 +25,9 @@ import Data.Char
 import Common.Annex
 import qualified Annex.Branch
 import Types.Remote
+import Logs
 import Logs.UUIDBased
 
-{- Filename of remote.log. -}
-remoteLog :: FilePath
-remoteLog = "remote.log"
-
 {- Adds or updates a remote's config in the log. -}
 configSet :: UUID -> RemoteConfig -> Annex ()
 configSet u c = do