Add subject to dummy thread roots

Sometimes, when adding dummy roots, Gnus displays the thread subject
on the first message:

(emacs-devel, fetched from archive.lwn.net)
                     ┌◈
    10:31            ├► martin rudalics          Re: emacs rendering comparisson between emacs23 and emacs26.3
    14:07            │├► Dmitry Gutov
    14:35            ││├► Dmitry Gutov
    14:45            ││╰► martin rudalics
    13:47            │╰► Alan Mackenzie
    14:45            ┆ ╰► martin rudalics

And sometimes it doesn't:

(bug-gnu-emacs, fetched from news.gmane.io)
                     ┌◈
    02:08            ├► Juri Linkov
    05:33            ╰► Stefan Monnier

Adding %S to the dummy line-format will cause the subject to appear
twice in the first situation (on the dummy root, then on the first
message); not great, but better than not knowing what the topic is.

0 files changed, 0 insertions, 0 deletions