If you have come this far, you probably have seen that this 'util'
pointer is used for many different purposes. Some are not even
contained in a command code, but buried deep in common code with no
clue who will use it and how.

The move to using commit-slab gives us a much better picture of how
some piece of data is associated with a commit and what for. Since
nobody uses 'util' pointer anymore, we can retire so that nobody will
abuse it again. commit-slab will be the way forward for associating
data to a commit.

As a side benefit, this shrinks struct commit by 8 bytes (on 64-bit
architecture) which should help reduce memory usage for reachability
test a bit. This is also what commit-slab is invented for [1].

[1] 96c4f4a3 (commit: allow associating auxiliary info on-demand -
2013-04-09)
Signed-off-by: NJunio C Hamano <gitster@pobox.com>