title: github - checked out from wrong branch
body: <p>I have two branches, my master A, my staging B, and my changes, C.</p>
  
  <p>I branched C from B and made various commits, however I now need to merge in all changes on branch C into master rather than merging in all of B itself.</p>
  
  <p>Is there an easy way to do this? I've checked stack overflow and apparently git checkout will only work if the base files are identical, which in this case they aren't...</p>