cat: replace match.bad() monkey patching with match.badmatch()

No known issues with the previous code since it restored the original method,
but this is cleaner.

mercurial/cmdutil.py

@@ -2437,22 +2437,16 b' def cat(ui, repo, ctx, matcher, prefix, '
             return 0
 
     # Don't warn about "missing" files that are really in subrepos
-    bad = matcher.bad
-
     def badfn(path, msg):
         for subpath in ctx.substate:
             if path.startswith(subpath):
                 return
-        bad(path, msg)
-
-    matcher.bad = badfn
-
-    for abs in ctx.walk(matcher):
+        matcher.bad(path, msg)
+
+    for abs in ctx.walk(matchmod.badmatch(matcher, badfn)):
         write(abs)
         err = 0
 
-    matcher.bad = bad
-
     for subpath in sorted(ctx.substate):
         sub = ctx.sub(subpath)
         try: