oe-build-perf-report: Update chart tooltip and chart type

- Update chart tooltip format to show value as size in MB for 'rootfs size'
and timestamp for 'tmpdir size'
- Add commit number to tooltip
- Update chart type to 'step chart' instead of 'line chart'

Signed-off-by: Ninette Adhikari <ninette@thehoodiefirm.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>