[PATCH 0/3] Convert run_status to a builtin

[Jeff King <peff@peff.net>]

This series of patches converts the run_status function from
git-commit.sh to a C builtin. Hopefully we can use this as a starting
point for further improvements (the only feature I added was
colorization).  I haven't had a chance to look closely at the
simultaneous traversal code in pu, but that would be the nice next place
to take this concept.

Patch 1/3 adds the necessary diff callback infrastructure, which may be
of value regardless of whether the final patch is accepted.

Patch 2/3 lib-ifies the diff color support so it can be reused by
run-status. Again, I think this is worth doing anyway from a code
cleanup standpoint.

Patch 3/3 does the actual conversion.

-Peff