Get the statistics from numstat instead of diff

Get the statistics from numstat instead of diff

The option --numstat of git log gives the statistics of
lines added and removed file.  Hence, it is not necessary
to parser a raw diff.

Another benefit, it is a less verbose log to be processed.
This also prepares the code for counting the changes per
file type.

Signed-off-by: Germán Póo-Caamaño <gpoo@gnome.org>