DC Migration - export/import results in missing pull request links on the target instance

Issue Summary

Pull request links are missing after a DC Migration (Server to Server)

Steps to Reproduce

1. Prepare two Bitbucket instances (source and target)
2. Create a Pull Request on the source instance and merge it
3. Navigate to the merge commit created from the PR.  The PR link is visible.
4. Export all projects from the source instance
5. Import the projects to the target instance
6. On the target instance, navigate to the merge commit created from step#2. The PR link is missing.

Expected Results

PR link should be visible, similar to the source.

Actual Results

PR link is missing

Workaround

1. Stop the target instance
2. Delete the following file in the target instance

   {BITBUCKET_HOME}/shared/config/upgrades/core-backfill-pull-request-commits

3. OPTIONAL: Enable DEBUG logging for

   com.atlassian.stash.internal.pull.PullRequestCommitBackfillJob

4. Start the target instance