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>

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