track total repo LoC over time

[mpdehaan]

The sum of lines added + lines_removed from the earliest commit date to the current date of any rollup interval should give the lines of code count at any point in time. We can call this repo_line_size.

Similarly, once we track file deletions, we should be able to track repo_file_size, but can't quite yet.

This graph would be interesting to show when comparing one repo with another.

This statistic can't be done per author, because another author may delete the lines from another, at least, not without running git blame a ton, which is too expensive to bother with at the present time.