...
From the GitHub Mirror
git clone httphttps://git-wip-us.apache.org/repos/asfgithub.com/apache/incubator-nifi.git
From your GitHub Fork
git clone git@github.com:<account name>/incubator-nifi.git
...
...
git clone httphttps://git-wip-us.apache.org/repos/asfgithub.com/apache/incubator-nifi.git
git clone git@github.com:<account name>/incubator-nifi.git
...