diff -r 023ad45e2fd2 -r fff21278554b mercurial/cmdutil.py
--- a/mercurial/cmdutil.py	Thu Dec 12 22:30:59 2019 -0800
+++ b/mercurial/cmdutil.py	Thu Dec 12 15:55:33 2019 -0800
@@ -261,7 +261,10 @@
 
 
 def check_at_most_one_arg(opts, *args):
-    """abort if more than one of the arguments are in opts"""
+    """abort if more than one of the arguments are in opts
+
+    Returns the unique argument or None if none of them were specified.
+    """
     previous = None
     for x in args:
         if opts.get(x):
@@ -270,6 +273,7 @@
                     _(b'cannot specify both --%s and --%s') % (previous, x)
                 )
             previous = x
+    return previous
 
 
 def check_incompatible_arguments(opts, first, *others):