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>

unpack-trees.c

@@ -1004,7 +1004,7 @@ int oneway_merge(struct cache_entry **src, struct unpack_trees_options *o)
 
 	if (old && same(old, a)) {
 		int update = 0;
-		if (o->reset) {
+		if (o->reset && !ce_uptodate(old)) {
 			struct stat st;
 			if (lstat(old->name, &st) ||
 			    ie_match_stat(o->src_index, old, &st, CE_MATCH_IGNORE_VALID))