diff-files: mark an index entry we know is up-to-date as such

diff-files: mark an index entry we know is up-to-date as such

This does not make any difference when running diff-files alone, but if
you internally run run_diff_files() and then run other operations further
on the index, we do not have to run lstat(2) again on entries we already
have checked.

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