--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

Messages.hs

@@ -300,13 +300,14 @@ jsonOutputEnabled = withMessageState $ \s -> return $
 		JSONOutput _ -> True
 		_ -> False
 
-explain :: Maybe RawFilePath -> StringContainingQuotedPath -> Annex ()
-explain Nothing _ = return ()
-explain (Just f) msg = do
+explain :: ActionItem -> Maybe StringContainingQuotedPath -> Annex ()
+explain ai (Just msg) = do
 	rd <- Annex.getRead id
 	when (Annex.explainenabled rd) $
-		outputMessage JSON.none id $
-			"[" <> QuotedPath f <> " " <> msg <> "]\n"
+		let d = actionItemDesc ai
+		in outputMessage JSON.none id $
+			"[" <> (if d == mempty then "" else (d <> " ")) <> msg <> "]\n"
+explain _ _ = return ()
 
 {- Prevents any concurrent console access while running an action, so
  - that the action is the only thing using the console, and can eg prompt