bug#10081: 23.3; Rmail does not properly update mode line when expunging messages if all messages are now deleted [MINOR]

[Mark Lillibridge]

    Visit an Rmail inbox with some messages.  In the mode line, you
should see something like "(RMAIL 2/2)".  Delete all the messages via
'd'.  The mode line changes to something like "(Rmail 2/2 deleted)".
Now hit 's'.  Although your messages are indeed expunged and an empty
inbox is saved, the mode line is not changed -- mine still shows "(Rmail
2/2 deleted)" even though the inbox contains no messages and certainly
not any deleted messages.

    If you kill off the buffer and then revisit it, the mode line
correctly shows "(Rmail)", which is what Rmail is supposed to be showing
for an empty inbox.

- Mark