title: Discard changes in one single line
body: <p>I've noticed that in Tower (Git client for the Mac) the user can discard changes even line by line. I wonder how could this be done using the command line? or maybe is something special of Tower?</p>
  
  <p>I frequently find myself in this case:</p>
  
  <pre><code>@@ -391,7 +392,7 @@ extern BOOL validateReceiptAtPath(NSString *path);
  
         NSURL *url = [self fileURL];
          if (url != nil) {
                  NSRect readFrame = [self _readPreferenceOfFileAtURL:url];
  -               
  +
                  for (NSScreen * screen in [NSScreen screens]) {
                          NSRect screenVisibleRect = [screen visibleFrame];
                          ...
  </code></pre>
  
  <p>See how I have one <code>+</code> and one <code>-</code>  ? I would like to discard it so my commit has the minimum changes (hence less possibilities of conflicts and easier review)  </p>
  
  <p>:)</p>