cat: use ctx.manifestnode() in place of ctx._changeset[0]

mercurial/cmdutil.py

@@ -2402,7 +2402,7 def cat(ui, repo, ctx, matcher, prefix,
     if len(matcher.files()) == 1 and not matcher.anypats():
         file = matcher.files()[0]
         mf = repo.manifest
-        mfnode = ctx._changeset[0]
+        mfnode = ctx.manifestnode()
         if mf.find(mfnode, file)[0]:
             write(file)
             return 0