Add diffstat of whole patch series to cover mail

Add diffstat of whole patch series to cover mail

Note: The diffstat is computed from the bottom of the first patch to
the top of the last, which gives a correct result if and only if the
series is contiguous. This is often the case, and handling anything
but this case is nontrivial, as it would involve creating a temporary
series of commits that are contiguous, with all the performance and
merge problems that would entail.

Signed-off-by: Karl Hasselström <kha@treskal.com>