[swarming] Fix bug introduced in 945d5d7da6b9f32a952.
[swarming] Fix bug introduced in 945d5d7da6b9f32a952.

This very rarely occurs, only during a very short window where the negative
cache was not caught, and the item was not detected as reapeable even in the
index.

TBR=iannucci@chromium.org

Bug: 781021
Change-Id: I1f4977fff59ac95ca81c6a2053da77d3d872ff52
Reviewed-on: https://chromium-review.googlesource.com/1007255
Reviewed-by: Marc-Antoine Ruel <d2bf0fc09b08f7b0888b3ddab32b3e89c7122c8b@chromium.org>
Commit-Queue: Marc-Antoine Ruel <d2bf0fc09b08f7b0888b3ddab32b3e89c7122c8b@chromium.org>

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py