Fix: Update createBranch path with a new value

What does this MR do and why?

Contributes to #299500 (closed)

Problem

Create merge request button on issue page does not work when user provides / in the branch name.

frontend sends a default branch name to the backend instead of the value that user has provided.

How to set up and validate locally

1. Open "Issues" in sidebar for a project
2. Locate an existing issue or create one from scratch
3. Click on disclosure arrow next to "Create merge request"
4. Choose "Create merge request and branch"
5. Type in a new branch name containing a slash, like "ABC/XYZ"
6. Choose a source branch name containing a slash, like "release/1.0.0"
7. Click on "Create merge request button"
8. Merge request and branch will be created
9. Branch name is the default suggested branch name instead of the one typed in
10. The "merge target" is also the default suggested branch ("master" or "main") instead of the one selected

MR acceptance checklist

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