Before support for commit histories was added, it was sufficient to

check if a change description had any modified fields to determine if

an actual change was present. However, the DiffField is marked as

changed if there is a new diff available. In the case of a squashed

review request, this is sufficient because the API will not allow you

to upload an identical diff. However, with multiple commit review

requests, where each commit is uploaded individually, it becomes more

difficult because we do not know the number of commits before

publishing is attempted.
To address this issue, we now determine if only the DiffField has

been marked as changed and current DiffSet has diff commits. In this

case, we know that the DiffCommits haven't changed and therefore the

publish should not be allowed.
Added a unit test to capture this behaviour.
We now also use fewer database queries in the case where the history

was previously squashed and no longer is.

Ran units tests.
Created a review request with commit history. Upadated the review

request with the same set of commits and tried to publish the review

request draft, but was not able to.
The provided unit test fails without the patch and passes with it.