Unable to merge re-opened Pull Request

Issue Summary

When attempting to merge a pull request that was declined and reopened, if a PR was merged in the meantime you are unable to merge it.

Steps to Reproduce

1. Create commit in Branch-1
2. Create commit in Branch-2
3. Create commit in Branch-3
4. Create pull request (PR1) to merge Branch-1 to existing branch (Target Branch)
5. Create pull request (PR2) to merge Branch-2 to existing branch (Target Branch)
6. Create pull request (PR3) to merge Branch-3 to existing branch (Target Branch)
7. Merge PR1 while PR2 and PR3 are open.
8. Decline PR2.
9. Merge PR3 while PR2 is declined.
10. Reopen PR2.
11. Attempt to Merge PR2

Expected Results

You are able to merge PR2 without issue

Actual Results

Receive the error "New changes were pushed to BRANCH in PROJ/repo while the merge was being performed. Please retry the merge"

Declining and reopening the pull request does not resolve the issue

Workaround

Option 1: Rebase

• Rebase the pull request using Bitbucket Server's built-in rebase functionality.
• Click on the "meatball" menu next to the Merge button in the top right of the pull request view
• Select the rebase option
  
• Review your choice in the "Rebase source branch" dialog and click the "Rebase" button.
  
• You can now merge your pull request.

Option 2: Delete / recreate

• Decline or delete the pull request
  (N.B. choose decline if you would like to keep the review history and comments)
• Recreate a pull request between the same branches.
• You can now merge your pull request.