bug#22694: 25.0.91; dired-mark-files-containing-regexp read file disk

[Tino Calancha <f92capac@gmail.com>]

> Isn't this by design?  Dired doesn't by default re-sync with the disk;
> if you don't like this, you can turn on auto-revert in the Dired
> buffer.
I don't like reverting the buffer either.
I found better to ignore the buffers visiting and handle all files with
with-temp-buffer. Using grep is also nice, but it would change the type of 
regexp.

Regardless of whether auto-revert is enabled or not, the doc. string 
of this function says that it marks files whose contain match a regexp,
so, it should check the file instead of relying in the visiting 
buffer.