bug#7906: keep the buffer still in rmail mode

[hf]

Hello,

I am using Emacs 23.2.1 on FreeBSD 7.1.  When I tried to receive incoming
emails by typing `g' under the rmail mode, the entire buffer of emacs
blinked (it acts the same on the X Window and the text-only terminal),
this is not so nice.  In the earlier version, only the echo area prompts
some text, and the main buffer is refreshed only when a new email comes.

Maybe this is not a real bug, but I believe keep it works as the old
way is much better.

Thanks,
Hong Feng