diff-lib: simplify do_diff_cache()

diff-lib: simplify do_diff_cache()

Since 34110cd (Make 'unpack_trees()' have a separate source and
destination index, 2008-03-06), we can run unpack_trees() without munging
the index at all, but do_diff_cache() tried ever so carefully to work
around the old behaviour of the function.

We can just tell unpack_trees() not to touch the original index and there
is no need to clean-up whatever the previous round has done.

Signed-off-by: Junio C Hamano <gitster@pobox.com>