diff: plug leaks in dirstat

diff: plug leaks in dirstat

The array of dirstat_file contained in the dirstat_dir structure is
not freed after the processing ends.  Unfortunately, the member that
points at the array, .files, is incremented as the gather_dirstat()
function recursively walks it, and this needs to be plugged by
remembering the beginning of the array before gather_dirstat() mucks
with it and freeing it after we are done.

We can mark t4047 as leak-free.  t4000, which is marked as
leak-free, now can exercise dirstat in it, which will happen next.

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