improve match explanations

Using == and != proved too hard to read, so went with [TRUE] and [FALSE]
after the term. I would kind of liked to have used emojis for a green
check and red X, but probably that is too fancy to be a good idea.

Make --explain output be inside [ ] with whitespace around them, to
avoid it ending with eg "[FALSE]]" and to make it easier to visually
find the start of it.

Sponsored-by: Dartmouth College's DANDI project

Messages.hs

@@ -306,7 +306,7 @@ explain ai (Just msg) = do
 	when (Annex.explainenabled rd) $
 		let d = actionItemDesc ai
 		in outputMessage JSON.none id $
-			"[" <> (if d == mempty then "" else (d <> " ")) <> msg <> "]\n"
+			"[ " <> (if d == mempty then "" else (d <> " ")) <> msg <> " ]\n"
 explain _ _ = return ()
 
 {- Prevents any concurrent console access while running an action, so