Zip-Archive mode doesn't check if the archive has changed

[Dan Jacobson]

Zip-Archive mode doesn't check if the archive has changed on disk if
there still is a buffer of one of its members when bringing back that
buffer.
$ echo Q>m
$ zip a m
  adding: m (stored 0%)
$ emacs .
RET RET C-x b RET C-z
$ echo X>m
$ zip -o a m
updating: m (stored 0%)
$ fg
RET
we see a Q not an X