archive-mode gets confused if two archives have the same filename

[Chris Moore]

Here are steps to reproduce a bug where Emacs shows a file from a
different archive than the one I requested:

1. make 2 directories:

  $ mkdir /tmp/dir1 /tmp/dir2

2. in each directory make a file "file.txt", each with different content

  $ echo 111 > /tmp/dir1/file.txt
  $ echo 222 > /tmp/dir2/file.txt

3. in each directory make a .zip or file containing the local file.txt

  $ cd /tmp/dir1; zip file.zip file.txt
  $ cd /tmp/dir2; zip file.zip file.txt

4. visit the first archive in Emacs and view file.txt inside it:

  C-x C-f /tmp/dir1/file.zip RET f

5. this shows a buffer containing "111" as expected, but then:

  C-x C-f /tmp/dir2/file.zip RET f

6. this shows the same buffer containing "111", when I expected to see
a new buffer containing "222".

The same bug happens if you use jar instead of zip.  And I'm guess the
same also happens with .arc, .lzh and .zoo files, whatever they might
be.

Chris.