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>