commit: write cache-tree data when writing index anyway

In prepare_index(), we refresh the index, and then write it to disk if this changed the index data. After running hooks we re-read the index and compute the root tree sha1 with the cache-tree machinery. This gives us a mostly free opportunity to write up-to-date cache-tree data: we can compute it in prepare_index() immediately before writing the index to disk. If we do this, we were going to write the index anyway, and the later cache-tree update has no further work to do. If we don't do it, we don't do any extra work, though we still don't have have cache-tree data after the commit. The only case that suffers badly is when the pre-commit hook changes many trees in the index. I'm writing this off as highly unusual. Signed-off-by: Thomas Rast <trast@student.ethz.ch> Signed-off-by: Junio C Hamano <gitster@pobox.com>