Keep details in MR when changing target branch

Issue: #30016 (closed)

What does this MR do?

Keeps details in the Merge Request when changing the target branch.

Fill in Data & Description	Data persists when target branch changed

Testing Case

1. Go to Project Page > Merge Requests > New merge request
2. In the "Select source branch" drop down, select a random branch name
3. Click "Compare branches and continue" button
4. This will lead to the "New Merge Request" Form page
5. Fill in Title > "foo"
6. Fill in Description > "foobar"
7. Scroll down to "Target branch" and click "Change branches"
8. This will lead back "New Merge Request" Branch page
9. In the "Target branch", select a different target branch
10. Click "Compare branches and continue" button
11. This will lead back to the "New Merge Request" Form page
12. The data will still persist

• Title will contain "foo"
• Description will contain "foobar"

(Note: make sure select a different target branch that has never been selected before)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Closes #30016 (closed)