Cannot set branch in registry

[awvwgk]

Currently we cannot set the branch we want to fetch in the registry file.

The workaround is to exploit that branches and tags are handled sufficiently similar that we can just pretend that our branch is a tag in things will work. This workaround will be okay with fpm since we are currently not making a distinction between branches and tags in the fpm.

On the other hand, this behaviour can also be exploited to add commits from forks using the tag key in the registry, like I have done in the past here: https://github.com/fortran-lang/fpm-registry/pull/28/commits/4e4da029fbcd5bea6e469c1c9d06a1b8f525d429