I suggest to add diffs so that you can see what was changed between two submissions in the History tab. It can be really helpful to find bugs, especially with lengthy code.
Until this becomes a thing, I currently use diffchecker for this. It’s really easy to just ctrl-a, ctrl-c, ctrl-v both versions and done. looks nice as well.
What would be really great is to be able to checkout the submissions as a git project.
When you start a contest or puzzle you don’t always plan to spend a lot of time on it and you’d be happy to get a proper history.
Also, Sublime provides diff out of the box and Notepad++ has a file diff plugin.
Like Manwe mentions in his advice for contests, and like many of us already do, you can use git or any version control system to keep an history of your changes.
There is a good UI for git called ungit, which I find pretty clean and simple to use. It’s easy to visually see all the code changes, branches, and tag a commit with “Score: 23.3, rank #42” for instance.