There is basically no login area. Gitweb is the official web interface for the source tracker git. If you have permissions to push to the repository the gitweb page is showing, you could possibly add a file to the repository, when shown on the page will inject malicious javascript code for example.

If you run gitweb for repositories where only yourself (an people you trust) has those permissions, this vulnerability is rather harmless. But if you are running gitweb for a large FOSS project with a lot of committers, you should be aware of that issue.