Change the prefix for builtin diff generation.

Change the prefix for builtin diff generation.

It's silly, and it shouldn't matter, but every time I look at
the diffs, I ended up just worrying why "l/" and "k/" as the
prefixes.

Junio says it's a tribute to linux-kernel, but graciously also
said I can change it to something else. So make it "a/" and "b/"
until somebody else complains ;)