bug#6810: 23.2; dired-omit-mode makes dired slow

[Chong Yidong]

Leo <sdl.web@gmail.com> writes:

> `dired-do-redisplay' iterates over the marked files calling
> dired-update-file-line which triggers dired-after-readin-hook (see line
> 1132 in dired-aux). So that hook is being run for as many times as the
> number of files marked.
>
> Any objection to something like this:

Shouldn't this handle dired-before-readin hook too?