oneway_merge(): only lstat() when told to update worktree

Although the subject line of 613f027 (read-tree -u one-way merge fix
to check out locally modified paths., 2006-05-15) mentions "read-tree
-u", it did not seem to check whether -u was in effect. Not checking
whether -u is in effect makes e.g. "read-tree --reset" lstat() the
worktree, even though the worktree stat should not matter for that
operation.

This speeds up e.g. "git reset" a little on the linux-2.6 repo (best
of five, warm cache):

        Before      After
real    0m0.288s    0m0.233s
user    0m0.190s    0m0.150s
sys     0m0.090s    0m0.080s

Signed-off-by: Martin von Zweigbergk <martinvonz@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

unpack-trees.c

@@ -1834,7 +1834,7 @@ int oneway_merge(struct cache_entry **src, struct unpack_trees_options *o)
 
 	if (old && same(old, a)) {
 		int update = 0;
-		if (o->reset && !ce_uptodate(old) && !ce_skip_worktree(old)) {
+		if (o->reset && o->update && !ce_uptodate(old) && !ce_skip_worktree(old)) {
 			struct stat st;
 			if (lstat(old->name, &st) ||
 			    ie_match_stat(o->src_index, old, &st, CE_MATCH_IGNORE_VALID|CE_MATCH_IGNORE_SKIP_WORKTREE))