--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

Utility/Matcher.hs

@@ -215,9 +215,12 @@ introspect :: (a -> Bool) -> Matcher a -> Bool
 introspect = any
 
 {- Converts a [MatchResult] into a description of what matched and didn't
- - match. -}
-describeMatchResult :: (op -> Bool -> MatchDesc) -> [MatchResult op] -> String
-describeMatchResult descop = unwords . go . simplify True
+ - match. Returns Nothing when the matcher didn't contain any operations
+ - and so matched by default. -}
+describeMatchResult :: (op -> Bool -> MatchDesc) -> [MatchResult op] -> String -> Maybe String
+describeMatchResult _ [] _ = Nothing
+describeMatchResult descop l prefix = Just $
+	prefix ++ unwords (go $ simplify True l)
   where
  	go [] = []
 	go (MatchedOperation b op:rest) =