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/Trust.hs

@@ -27,14 +27,11 @@ import Common.Annex
 import Types.TrustLevel
 import qualified Annex.Branch
 import qualified Annex
+import Logs
 import Logs.UUIDBased
 import Remote.List
 import qualified Types.Remote
 
-{- Filename of trust.log. -}
-trustLog :: FilePath
-trustLog = "trust.log"
-
 {- Returns a list of UUIDs that the trustLog indicates have the
  - specified trust level.
  - Note that the list can be incomplete for SemiTrusted, since that's