bug#3471: 23.0.94; linum-mode and invisible lines

Please describe exactly what actions triggered the bug
and the precise symptoms of the bug:

In buffers where many many lines are invisible, enabling linum-mode
would take a long time and C-g won't be able to stop it.

More specifically, I use magit to manage a small git repo, its status
buffer at the moment has 11850 lines (buffer size 366524) with only <
100 lines visible. Running linum-mode on it would freeze emacs for a few
minutes on a intel core duo 2.0G machine. It looks like linum has wasted
a lot of time on invisible lines. Is that necessary?

Although I'm aware that I can work around it by setting
line-number-display-limit but my feeling is linum is doing something
inefficiently.

GNU Emacs 23.0.94.1 (i386-apple-darwin9.7.0, NS apple-appkit-949.46) of
 2009-05-23 on 200.sub-75-216-116.myvzw.com

Thanks.

Leo