title: Create a patch by comparing a specific branch on the remote with a specific local branch
body: <p>I've been working with git for a few weeks, but now I'd like to contribute back to this open source project. I've merged my work with the latest, remote by pulling editing out conflicts and it all looks the way it should in gitk. Now I need to create a patch that is against the latest version of origin (remote) master.
  So I thought the following command would work:</p>
  
  <p><code>git format-patch origin:master --stdout &gt; 25032011.patch</code> </p>
  
  <p>but I get:</p>
  
  <p><code>fatal: Invalid object name 'origin'.</code></p>
  
  <p>So I've obviously got the command wrong. So how would I create a patch by comparing a specific branch on the remote with a specific local branch? </p>