export --json: Fill in the file field

Like import was using ActionItemWorkTreeFile, it's ok to use it for export,
even though it might not correspond with a file in the work tree.
And renamed it to ActionItemTreeFile to make that clearer.

Note that when an export has to rename files, it still uses
ActionItemOther, so file will still be null in that case, but as no file is
being transferred, that seems ok.

Command/Whereis.hs

@@ -96,7 +96,7 @@ perform o remotemap key ai = do
 			mapM_ (showRemoteUrls remotemap) urls
 		Just formatter -> liftIO $ do
 			let vs = Command.Find.formatVars key
-				(AssociatedFile (actionItemWorkTreeFile ai))
+				(AssociatedFile (actionItemFile ai))
 			let showformatted muuid murl = putStr $
 				Utility.Format.format formatter $
 					M.fromList $ vs ++ catMaybes