Simplify topo-sort logic

.. by not using quite so much indirection.

This currently grows the "struct commit" a bit, which could be avoided by
using a union for "util" and "indegree" (the topo-sort used to use "util"
anyway, so you cannot use them together), but for now the goal of this was
to simplify, not optimize.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>