avoid redirection warning when cloning anonymously
Using `centpkg clone -a` results in a warning from git due to the URL
being redirected:
$ centpkg clone -a git
Cloning into 'git'...
warning: redirecting to https://gitlab.com/redhat/centos-stream/rpms/git.git/
...
Update anongiturl to point to the real URL for HTTP cloning. The
addition of the trailing slash does not raise a warning, so it may be
safely left off the URL.