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