tags-search doesn't check files are up-to-date

["Chris Moore" <christopher.ian.moore@gmail.com>]

I was just trying to find something in an old IRC logfile.  I went to
the directory holding the files with dired, marked the last week's
worth of logs and hit "A" (dired-do-search) to search them for the
string.

Emacs told me "All files processed" - it didn't find the string.

"grep" tells me differently - the string is there.

It turns out that I had visited one of the logs when it still being
written a few days ago.  Emacs had the first half of the file in a
buffer, and the string I wanted was in the 2nd half - on disk, but not
in the buffer.

I would expect Emacs to say "File ... changed on disk.  Reread from
disk? (yes or no) " when I try searching a file that's changed, but it
silently assumes I don't want to know that it is using an old copy.

It's pretty simple to reproduce:  make an empty file, visit it, append
'xxx' to it outside of Emacs, mark the file in dired and search it for
'xxx' using the "A" (dired-do-search) key.  It won't find the 'xxx'.