Scheduling release updates with no difference should not be possible.

[bhearsum]

Steps to reproduce:

While working on bug-1355477, I realized that It is possible to download the current version of a release and upload it as an update without making any changes to the file.

Actual results:

The update was accepted as a scheduled change for a release.

Expected results:

The system should have rejected scheduling of updates with no changes.

(Imported from https://bugzilla.mozilla.org/show_bug.cgi?id=1417355)