title: Git: Show all of the various changes to a single line in a specified file over the entire git history
body: <p>I've looked around, and am not sure if this is possible, but here goes:</p>
  
  <p>I have a (javascript) file (say /lib/client.js) in which I have a unique identifier assigned to a variable, like so:
     <code>var identifier = "SOME_IDENTIFIER";</code></p>
  
  <p>You can think of the identifier like a version number:  Periodically, we'll change this variable to a new identifier.</p>
  
  <p>What I'd like to do is find all of the unique identifiers we've ever used.  How can I do this with git?</p>
  
  <p>I imagine there might be a way to search through the git history, and print the line matching <code>"var identifier ="</code>.  I could de-dupe this list manually.</p>
  
  <p>Anyway, I'd appreciate any insight here.  Thanks.</p>