The scenario here is that:

• You've cloned someone else's repository from GitHub to browse or build their source;
• And you see something that you'd like to change;
• You 🔧 in your local repository on a branch;
• You attempt to push, but can't because you don't have permission to do so.

What you likely want to do is fork the origin repository to your own account, reconfigure your remotes (origin 👉 your fork; upstream 👉 the source fork?), then push -u your branch to your fork.

@haacked mentioned in Slack that this would be nice to have in Atom: github/VisualStudio#900