--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

Annex/Import.hs

@@ -762,7 +762,7 @@ importKeys remote importtreeconfig importcontent thirdpartypopulated importablec
 						warning (UnquotedString (show e))
 						return Nothing
 	
-	importordownload cidmap (loc, (cid, sz)) largematcher= do
+	importordownload cidmap (loc, (cid, sz)) largematcher = do
 		f <- locworktreefile loc
 		matcher <- largematcher f
 		-- When importing a key is supported, always use it rather
@@ -771,7 +771,7 @@ importKeys remote importtreeconfig importcontent thirdpartypopulated importablec
 		let act = if importcontent
 			then case Remote.importKey ia of
 				Nothing -> dodownload
-				Just _ -> if Utility.Matcher.introspect matchNeedsFileContent matcher
+				Just _ -> if Utility.Matcher.introspect matchNeedsFileContent (fst matcher)
 					then dodownload
 					else doimport
 			else doimport
@@ -781,7 +781,7 @@ importKeys remote importtreeconfig importcontent thirdpartypopulated importablec
 		case Remote.importKey ia of
 			Nothing -> error "internal" -- checked earlier
 			Just importkey -> do
-				when (Utility.Matcher.introspect matchNeedsFileContent matcher) $
+				when (Utility.Matcher.introspect matchNeedsFileContent (fst matcher)) $
 					giveup "annex.largefiles configuration examines file contents, so cannot import without content."
  				let mi = MatchingInfo ProvidedInfo
 					{ providedFilePath = Just f
@@ -994,14 +994,15 @@ addBackExportExcluded remote importtree =
  -}
 makeImportMatcher :: Remote -> Annex (Either String (FileMatcher Annex))
 makeImportMatcher r = load preferredContentKeylessTokens >>= \case
-	Nothing -> return $ Right matchAll
-	Just (Right v) -> return $ Right v
+	Nothing -> return $ Right (matchAll, matcherdesc)
+	Just (Right v) -> return $ Right (v, matcherdesc)
 	Just (Left err) -> load preferredContentTokens >>= \case
 		Just (Left err') -> return $ Left err'
 		_ -> return $ Left $
 			"The preferred content expression contains terms that cannot be checked when importing: " ++ err
   where
 	load t = M.lookup (Remote.uuid r) . fst <$> preferredRequiredMapsLoad' t
+	matcherdesc = MatcherDesc "preferred content"
 
 {- Gets the ImportableContents from the remote.
  -