This uses a normal c array to store parent sha's. That means that we
save a lot of room, as they are only 20 bytes rather than 40 + the cost of
an NSArray and the NSStrings.
This produces cleaner code and is also significantly faster
than the old obj-c code: linux-2.6 goes from ~8 seconds
to ~6 seconds.
Using the c++ code allows us to do easy optimizations
Pieter de Bie