I thought it would be neat if it were possible to take a Git repository, run some script, and have it generate the number of lines in the code base, and the proportion of each a
You probably need gitdm, it can do exactly what you need. We use it for Mahara project to produce contribution statistics.
Just do what README suggests:
A typical command line used to generate the "who write 2.6.x" LWN articles looks like:
git log -p -M v2.6.19..v2.6.20 | gitdm -u -s -a -o results -h results.html
You can also customise it for your own purposes.