use TopFilePath for associated files

Fixes several bugs with updates of pointer files. When eg, running
git annex drop --from localremote
it was updating the pointer file in the local repository, not the remote.
Also, fixes drop ../foo when run in a subdir, and probably lots of other
problems. Test suite drops from ~30 to 11 failures now.

TopFilePath is used to force thinking about what the filepath is relative
to.

The data stored in the sqlite db is still just a plain string, and
TopFilePath is a newtype, so there's no overhead involved in using it in
DataBase.Keys.

Annex/Drop.hs

@@ -19,6 +19,7 @@ import Annex.Wanted
 import Config
 import Annex.Content.Direct
 import qualified Database.Keys
+import Git.FilePath
 
 import qualified Data.Set as S
 import System.Log.Logger (debugM)
@@ -49,7 +50,7 @@ handleDropsFrom :: [UUID] -> [Remote] -> Reason -> Bool -> Key -> AssociatedFile
 handleDropsFrom locs rs reason fromhere key afile preverified runner = do
 	l <- ifM isDirect
 		( associatedFilesRelative key
-		, Database.Keys.getAssociatedFiles key
+		, mapM getTopFilePath <$> Database.Keys.getAssociatedFiles key
 		)
 	let fs = if null l then maybeToList afile else l
 	n <- getcopies fs