What's the difference between "git fetch <url>" and "git add remote upstream <url>" followed by "git fetch upstream"?

When you fetch with a URL, you also have to specify the <refspec> you want to fetch, i.e. the branch or tag, otherwise it will just fetch the default HEAD of the remote URL as FETCH_HEAD, which is probably not what you want.

The syntax is an optional + followed by <src>:<dst>. If you omit <dst>, FETCH_HEAD will be used.

So for example:

git fetch https://github.com/someuser/someproject refs/heads/master:upstream/master

which will create the upstream/master remote branch locally.

More advanced options are available.