--explain for preferred/required content matching

And annex.largefiles and annex.addunlocked.

Also git-annex matchexpression --explain explains why its input
expression matches or fails to match.

When there is no limit, avoid explaining why the lack of limit
matches. This is also done when no preferred content expression is set,
although in a few cases it defaults to a non-empty matcher, which will
be explained.

Sponsored-by: Dartmouth College's DANDI project

Command/Export.hs

@@ -491,8 +491,8 @@ filterExport :: Remote -> Git.Ref -> Annex (ExportFiltered Git.Ref)
 filterExport r tree = logExportExcluded (uuid r) $ \logwriter -> do
 	m <- preferredContentMap
 	case M.lookup (uuid r) m of
-		Just matcher | not (isEmpty matcher) ->
-			ExportFiltered <$> go (Just matcher) logwriter
+		Just (matcher, matcherdesc) | not (isEmpty matcher) ->
+			ExportFiltered <$> go (Just (matcher, matcherdesc)) logwriter
 		_ -> ExportFiltered <$> go Nothing logwriter
   where
 	go mmatcher logwriter = do