--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/MatchExpression.hs

@@ -1,6 +1,6 @@
 {- git-annex command
  -
- - Copyright 2016 Joey Hess <id@joeyh.name>
+ - Copyright 2016-2023 Joey Hess <id@joeyh.name>
  -
  - Licensed under the GNU AGPL version 3 or higher.
  -}
@@ -10,7 +10,6 @@ module Command.MatchExpression where
 import Command
 import Annex.FileMatcher
 import Utility.DataUnits
-import Utility.Matcher
 import Annex.UUID
 import Logs.Group
 
@@ -84,15 +83,14 @@ seek o = do
 				, configMap = M.empty
 				, repoUUID = Just u
 				}
-	case parsedToMatcher $ parser ((matchexpr o)) of
+	case parsedToMatcher (MatcherDesc "provided expression") $ parser ((matchexpr o)) of
 		Left e -> liftIO $ bail $ "bad expression: " ++ e
 		Right matcher -> ifM (checkmatcher matcher)
 			( liftIO exitSuccess
 			, liftIO exitFailure
 			)
   where
-	checkmatcher matcher = matchMrun matcher $ \op ->
-		matchAction op S.empty (matchinfo o)
+	checkmatcher matcher = checkMatcher' matcher (matchinfo o) S.empty
 
 bail :: String -> IO a
 bail s = do