Bitbucket importer: Fix for MRs not showing diffs after import

What does this MR do and why?

Falls back to ['merge_commit']['hash'] if ['source']['commit']['hash'] does not match any commits on the imported project after importing from Bitbucket (API reference).

This solves the issue when a pull request on Bitbucket was squashed before merge and the source branch deleted:

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

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

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.