measuring the % change between two commits

I'm trying to come up with a % measure for the changes between two commits.

I don't mind how precisely it is calculated, since this is only for a
thresholding purpose where we can adjust the threshold if the way it
is calculated is not what we think it should be.

The logging options that put out a % figure only do so for
copied/renamed files.  Merely modified files do not get a % figure
attached.

I could do a --numstat and then do a 'wc -l' on each file I guess, but
I was hoping to avoid that.

--dirstat gives you a percentage but does not count the top level directory.

Any ideas?

-- 
Sitaram