Fix extraneous lstat's in 'git checkout -f'

In our 'oneway_merge()' we always do an 'lstat()' to see if we might need to mark the entry for updating. But we really shouldn't need to do that when the cache entry is already marked as being ce_uptodate(), and this makes us do unnecessary lstat() calls if we have index preloading enabled. Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org> Signed-off-by: Junio C Hamano <gitster@pobox.com>