D5495: revset: add "branch" positional arguments to the merge revset

[Yuya Nishihara]

>   This would not make it possible to select multiple "merged with" branches by doing: hg log -r "merge(feature1, feature2)"
>   Instead I guess you are proposing that for that use case we force the user to do: hg log -r "merge('re:(feature1|feature2)')
>   
>   Did I understand you correctly?

Yes. And we can introduce other prefixes of string matcher if `re:` seemed
unnecessarily complex.