how to revert changes in working tree?

[Liu Yubao <yubao.liu@gmail.com>]

I'm confused how to revert changes in working tree:

$ git fetch
$ git merge "sync with origin" HEAD origin
....conflict....

$ git branch
* master
  origin

$ git status
# .....: needs update
# .....: needs update
(In fact I never modified anything in this tree, and "git diff"
showed many difference indeed, very strange).

I tried "git update-index --refresh", "git reset --hard",
"git reset --hard master", "git checkout master", 
"git checkout -f master", but "git status" still said same
as above.

At last, I deleted all files that were reported to be updated
with "rm -rf", ran "git checkout master" and "git status", then
git reported:
#       deleted:    ....
#       deleted:    ....