Avoid filling target branch when unknown and source is default branch

What does this MR do?

In the merge request compare page, we will avoid prefilling the target branch in the following scenario:

• The source and target projects are the same
• The target branch is not set in the params
• The source branch is the repository default branch

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/62807

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• Code review guidelines
• Merge request performance guidelines
• Style guides