[Speed] speed.python.org: move to Git, remove old previous results

[Victor Stinner]

Hi,

I'm going to remove old previous benchmark results from
speed.python.org. As we discussed previously, there is no plan to keep
old results when we need to change something. In this case, CPython
moved from Mercurial to Git, and I'm too lazy to upgrade the revisions
in database. I prefer to run again benchmarks :-)

My plan:

* Remove all previous benchmark results
* Run benchmarks on master, 2.7, 3.6 and 3.5 branches
* Run benchmarks on one revision per year quarter on the last 2 years
* Then see if we should run benchmarks on even older revisions and/or
if we need more than one plot per quarter.
* Maybe one point per month at least? The problem is that the UI is
limited to 50 points on the "Display all in a grid" view of the
Timeline. I would like to be able to render 2 years on this view.

For each year quarter, I will use the first commit of the master
branch on this period.

Victor