make MatchFiles introspectable

matchNeedsFileContent is not used yet, but shows how to add information
about terminals. That one would be needed for
https://git-annex.branchable.com/todo/sync_fast_import/

Note the tricky bit in Annex.FileMatcher.call where it folds over the
included matcher to propagate the information.

This commit was sponsored by Svenne Krap on Patreon.

Command/MatchExpression.hs

@@ -91,7 +91,8 @@ seek o = do
 			, liftIO exitFailure
 			)
   where
-	checkmatcher matcher = matchMrun matcher $ \a -> a S.empty (matchinfo o)
+	checkmatcher matcher = matchMrun matcher $ \op ->
+		matchAction op S.empty (matchinfo o)
 
 bail :: String -> IO a
 bail s = do