title: Untrack tracked file in Git -- but only in specific branch?
body: <p>Originally, I had everything contained in my master branch.</p>
  
  <p>Then, I decided to split into Mac and Windows branches, and have the master branch only track the source code files. So I began to only stage changes to source code files when in the master branch, and merge those source code changes with the Mac and Windows branches as needed.</p>
  
  <p>Problem is, when I originally tracked everything in my master branch, I had tracked the project file as well. Thus, I would be adding new files to the project and getting it compiling in Mac and Windows, but upon switching back to the master branch, the project file would be switched back to the old version as well, causing compilation to fail.</p>
  
  <p>So how can I stop tracking my project file in the master branch <em>now</em>? I don't want to stop tracking it in my Mac or Windows branches, and I don't want to remove it from earlier versions  in the master branch either.</p>