project.config: update Code-Review copyCondition

Add 'has:unchanged-files'. This allows the vote to be copied if the set
of files is not changed; the contents are allowed to change [1]. This
matches the configuration used on chromium.googlesource.com/All-Projects
and matches the behavior of libvpx.

[1]: https://gerrit-review.googlesource.com/Documentation/config-labels.html#label_copyCondition

Change-Id: I4b0b0301d5b61d3c5736bbd5172e9f9f9e6db78e