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 introduces a new object, PBGitLane that keeps track of the current lane.
We used to only need a sha for a lane, but now that more information is
needed, an extra object is in order. PBGitLane keeps a lane index number. This
number is later used to pick a color.
Benjamin Kramer