Fix serialization of pull_requests in Bitbucket Server Import

What does this MR do and why?

Add missing source_branch_sha to hash serialization

Related to: #411534 (closed)

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Set up a local Bitbucket Server instance docs.
2. Create a repo, and a PR with comments in Bitbucket
3. Enable local requests in http://gdk.test:3000/admin/application_settings/network
4. Import a new project /projects/new#import_project
5. Choose Bitbucket Server
6. Enter your server details, and click List your Bitbucket Server repositories
7. Next to one of your BitBucket repositories, click Import
8. You can tail -f log/importer.log to watch the import progress
9. The import should finish and you should have a new project imported with a repo, MR, and comments on the MR

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.