Resolve "Shortcut to create merge request from Web IDE"

What does this MR do?

Before the user had to choose between committing to a new branch, committing to a new branch AND creating an MR, or committing to the current branch regardless of whether or not it already has an MR.

This commit separates the creation of an MR from whether or not they commit to an existing or new branch

What are the relevant issue numbers?

Closes #57482 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• 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
• Security reports checked/validated by reviewer

EE Port

https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/10601