Fix Bitbucket import

What does this MR do?

In https://gitlab.com/gitlab-org/gitlab-ce/commit/ebf16ada856efb85424a98848c141f21e609886a we introduced a SHA validator to ensure that the data provided in merge request diffs when importing, was legit. Nevertheless, the validator assumed that the SHA should be 40 chars long.

When we import a project from BitBucket, the retrieved SHA is shorter (12 chars long). Therefore, this validator prevented to create a valid MergeRequestDiff for ever MergeRequest (triggering an exception).

What are the relevant issue numbers?

Closes #58804 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer