cmdutil: return underscore-separate name from check_at_most_one_arg()

As noticed by Yuya, when I changed the function (during review) to
work with underscore-separated names as input, I forgot to make sure
the returned name was also underscore-separated. We don't have any
cases where it matters yet, but it should still clearly be fixed.

Instead of converting the hyphen-separated value we already have in
`previous`, I'm changing it so we convert to the underscore-separated
values to be hyphen-separated only when we need to display them. This
will also help a coming change where we allow the inputs to native
strings instead only bytes.

Differential Revision: https://phab.mercurial-scm.org/D7698

mercurial/cmdutil.py

@@ -265,13 +265,17 b' def check_at_most_one_arg(opts, *args):'
 
     Returns the unique argument or None if none of them were specified.
     """
+
+    def to_display(name):
+        return name.replace(b'_', b'-')
+
     previous = None
     for x in args:
         if opts.get(x):
-            x = x.replace(b'_', b'-')
             if previous:
                 raise error.Abort(
-                    _(b'cannot specify both --%s and --%s') % (previous, x)
+                    _(b'cannot specify both --%s and --%s')
+                    % (to_display(previous), to_display(x))
                 )
             previous = x
     return previous