The method TestPullRequest#diff sometimes returns wrong information

Considering the following steps:

1. create a commit A at the source branch
2. create a PR from source branch to target branch (assume the head commit of the target branch is B)
3. create a new commit C at the target branch

Ideally, we want the diff files of the PR to be the diff files of commit A.
But the method TestPullRequest#diff returns the diff files of both A and reversed C.

I found this issue when writing the test case for SKARA-1385. As a workaround, I always create a new PR when a new commit is pushed to the target branch.