Viewing 'Merged Pull reuqests' tab in Compare page shows an error when there is a merged pull request (BB-15151)

Summary

When a user compares two tags or two branches in a repository, the 'Merged pull requests' tab shows an error if there is a merged pull request.
If there is no merged pull request, then no error occurs, the user sees an empty list.

Steps to Reproduce

1. Create a new repository in Bitbucket.
2. Clone the repository locally.
3. Create a file.
4. Commit the file >> Add a tag V1 to this commit.
5. Create a new branch called test_branch.
6. Checkout the new branch.
7. Create a second file.
8. Commit file.
9. Push commits and tags to Bitbucket.
10. Create a PR to merge test_branch into master.
11. Merge the above PR.
12. Pull changes locally.
13. Add a tag V2 to the merge commit.
14. Push tags to Bitbucket.
15. In Bitbucket, go to Compare.
16. Compare master and test_branch.
17. Compare tag V1 and tag V2.
18. For both steps, 16 and 17, check 'Merged pull requests' tab.

Expected Results

The tab should show a list of the merged pull requests.

Actual Results

The tab shows:

Error

We were unable to load this page

Notes

This occurs in both Git and Mercurial repos.

May be related to this: Issue 11273