Avoid unpacking packages twice when comparing .changes. (Closes: #843531)

If the parent container of this .buildinfo is a changes file, we should ignore
members that are referenced in both. This avoids recursing into files twice
where a .buildinfo references a file that is also listed in that member's
parent .changes file:

  foo.changes → foo.deb
  foo.changes → foo.buildinfo → foo.deb

Signed-off-by: Chris Lamb <lamby@debian.org>