Add count difference graphs in addition to current absolute value graphs

[asharov]

There was some discussion in issue #12 after it was closed to "ignore the first commit". Apparently the idea was to plot only the changes that had happened, ignoring the counts of the first commit. This makes sense, especially when the first commit was from a shallow clone or imported from somewhere else, when the commit maker incorrectly gets all of that commit assigned to themselves.

Making this kind of graph would not be too much work on top of the existing code. But it seems like a better idea to first refactor the summary data gathering to its own testable functions and only then implement this on top of that.