how to revert changes in working tree?

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: ....