write_index(): optimize ce_smudge_racily_clean_entry() calls with CE_UPTODATE

write_index(): optimize ce_smudge_racily_clean_entry() calls with CE_UPTODATE

When writing the index out, we need to check the work tree again to see if
an entry whose timestamp indicates that it could be "racily clean", in
order to smudge it if it is stat-clean but with modified contents.

However, we can skip this step for entries marked with CE_UPTODATE,
which are known to be the really clean (i.e. the one we already have
checked when we prepared the index).  This will reduce lstat(2) calls
necessary in git-status.

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