prevent trust commands from trying to do things in a bare repo

Since they need to stage changes, they would actually, if allowed to run,
succeed, but wipe out existing trust.log content.

Command.hs

@@ -10,7 +10,7 @@ module Command where
 import Control.Monad.State (liftIO)
 import System.Directory
 import System.Posix.Files
-import Control.Monad (filterM, liftM)
+import Control.Monad (filterM, liftM, when)
 import System.Path.WildMatch
 import Text.Regex.PCRE.Light.Char8
 import Data.List
@@ -104,6 +104,13 @@ isAnnexed file a = do
 		Just v -> a v
 		Nothing -> return Nothing
 
+notBareRepo :: Annex a -> Annex a
+notBareRepo a = do
+	g <- Annex.gitRepo
+	when (Git.repoIsLocalBare g) $ do
+		error "You cannot run this subcommand in a bare repository."
+	a
+
 {- These functions find appropriate files or other things based on a
    user's parameters, and run a specified action on them. -}
 withFilesInGit :: CommandSeekStrings