Apply Git reference fix to new Code dropdown

MR: Automatically select correct git reference base... (!196571 - merged)

The regression was discovered when we switched to the new Code dropdown. See the thread: #473446 (comment 2532587306)

Originally the issue was fixed in the old code dropdown, while we were already working on the new implementation.

MR with the fix: !177680 (merged)

Original issue description

The "Git reference" in the "New workspace" form is always prefilled as the default branch, no matter whether the user triggered this form from a page that represents a completely different git reference. That's inconsistent with how the Web IDE works.

Solution

When the "New workspace" form is triggered from any page within a project, it should use the currently selected git reference on the page they are on.