Fix bug where create MR toggle wasn't inspected correctly

What does this MR do and why?

This MR fixes a bug where the Start a new merge request with these changes toggle was ignored when uploading a new file.

Related: Files not tracked with LFS when uploaded throug... (#326342 - closed)

How to set up and validate locally

Using master branch

From the default branch of a test project:

1. Click +
2. Select Upload file
3. Disable the Start a new merge request with these changes toggle
4. Click the Upload file button
5. Observe you are now on a 'create new MR page'

Using this branch (ashmckenzie/fix-start-mr-toggle-ignored)

Repeat steps 1-4. For step 5., instead of being on the 'create new MR page', you should now looking at the newly uploaded file, in a new branch instead.

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.