Automatically select correct git reference based on referrer route

What does this MR do and why?

When the "New workspace" form is triggered from any page within a project (e.g. /tree/v1.0.0), it now automatically uses the currently selected git reference on the page they are on. That's making it consistent with how the Web IDE or Single File Editor works.

Screenshots or screen recordings

Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Ensure it works with branches
2. Ensure it works with tags