dired-do-compress doesn't ask before overwriting

[Dan Jacobson]

Z runs the command dired-do-compress, which, if one has both
access_log.gz and access_log in the same directory, and hits Z on
access_log, won't ask about overwriting access_log.gz.  I didn't try
what would happen if I hit Z upon access_log.gz but I bet it also
wouldn't ask.  Sure it asks one question always, but it should ask a
second question if there is the possibility of unintentional
overwriting.
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