I already did the git rebase --abort as soon as it started complaining about something I new I didn't change.

What if master has changed since I started the branch? Should the command instead be 'git rebase --onto arc3.master mybranch~n mybranch'? Or does it not matter where the head of master is relative to the beginning of my branch? Should I rebase to the head of master first, and then move it over to arc3.master?