diff -r 315f537627c1 -r 4bddc2f72879 mercurial/scmutil.py
--- a/mercurial/scmutil.py	Fri Apr 05 01:17:15 2019 +0200
+++ b/mercurial/scmutil.py	Sat Apr 13 01:53:59 2019 +0200
@@ -2313,3 +2313,13 @@
         mark,
         mark,
     )
+
+
+def ismember(ui, username, userlist):
+    """Check if username is a member of userlist.
+
+    If userlist has a single '*' member, all users are considered members.
+    Can be overridden by extensions to provide more complex authorization
+    schemes.
+    """
+    return userlist == [b'*'] or username in userlist