ls-files: move only kept cache entries in prune_cache()

ls-files: move only kept cache entries in prune_cache()

prune_cache() first identifies those entries at the start of the sorted
array that can be discarded.  Then it moves the rest of the entries up.
Last it identifies the unwanted trailing entries among the moved ones
and cuts them off.

Change the order: Identify both start *and* end of the range to keep
first and then move only those entries to the top.  The resulting code
is slightly shorter and a bit more efficient.

Signed-off-by: Rene Scharfe <l.s.r@web.de>
Reviewed-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>