I think you have two problems here. First, why should you purely rely on the diff when most systems allow you to enter revision comments? Like good code comments, you discover why the change was made and not just the change itself.
Second, if you have this capability, make it a good practice to put all of them in the same place. There isn't any need to look through the file for marked out lines of code that are no longer needed. Comments inside working code are there to tell you why it is coded this way.
Once you put this into practice, the habits formed make the code base easier to work on for everyone.
Associated bug and feature tracking along with why you're changing this file, can give you an idea about how deep you need to dig into the history and possibly looking at the diffs. I had a request to "Change back to the original formula." I knew right where to go within the revision history and only reviewed one or two diffs.
Personally, remarked out code looks like a work in progress for a problem that is being solved by trial and error. Get this mess out of production code. Being able to easily slip lines of code in and out only makes it easier to be confused.