More prevention of duplicate builds across multiple updates (#682)
Add a unit test to ensure that we disallow adding duplicate builds when editing updates
Don't try and set the request of an update if it's already in that state.

This can occur when editing updates, and causes certain tests to fail. Since
PackageUpdate.set_request throws an exception if it is already in the requested
state, we want to avoid this because at this point we've already made changes
to the update/builds.
Merge pull request #30 from fedora-infra/feature/ticket-682

Disallow the same build in two different updates