There are redirects implemented to translate paths from our git repositories to diffusion callsigns. That's because we have templates on mediawiki.org linking to git repositories, like Template:WikimediaDownload, based on the extension name.
For example, on the Extension:Echo page, there's a link to the repo summary pointing to https://phabricator.wikimedia.org/r/project/mediawiki/extensions/Echo/, which redirects to https://phabricator.wikimedia.org/diffusion/ECHO/
The problem here is that, while the repository URL is public, the URL prior to the redirect is not, displaying a login form for people not logged in. Hence, for now the link to git.wikimedia.org was restored: https://www.mediawiki.org/w/index.php?title=Template:WikimediaDownload&diff=prev&oldid=1995153
I think this should be a simple matter of permissions under the /r/project/ path.