feat: add chart for commit count

[ninetteadhikari]

Issue

The problem is we triggered a test of an older revision to narrow down when performance changed. The "start_time" in the data is when the test is run, not when the commit was merged and this leads to the 'fun' graphs.
We can add in data about when the commit was made however when I do that, we face a new issue. The issue is that git's timestamps are granular to 1s. We'll usually merge a set of commits at the same time so they will all have the same timestamp for a block of them.
This means that even if we use the commit date, all the points can't be distinguished on the graph.
The author date doesn't work either as the commits are not merged in author date order.
The most accurate measurement is the commit count, which always increases with time however we rightly concluded that a sense of time on the charts is also of key importance.
I worked out how to change the charts to plot commit number instead of start time, but then we lose the date data.

Possible solution

• Switch back to commit_count chart
• Add the commit_count chart as a separate tab next to the start_time chart