git over https implementation depends on the user agent starting with "git"

I'm trying to write a javascript based git client that lives inside a browser extension. If I use a user agent that starts with "git", my client works. If it doesn't start with "git", bitbucket returns a 404 for git http endpoints.

The problem is that I can't set the user-agent of an xhr in production. I'm able to do it in development by intercepting the request with a local proxy and changing it before I send.