scripts/buildperf: Add chart tabs for commit count/time

scripts/buildperf: Add chart tabs for commit count/time

We triggered a test of an older revision to narrow down when performance
changed. 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.

To solve this this patch adds the commit_count chart as a separate tab
next to the start_time chart

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>