Git pull into wrong branch

git reset --hard ORIG_HEAD 

From the git reset man page (if you just did the pull):

Undo a merge or pull

$ git pull                         (1)
Auto-merging nitfol
CONFLICT (content): Merge conflict in nitfol
Automatic merge failed; fix conflicts and then commit the result.
$ git reset --hard                 (2)
$ git pull . topic/branch          (3)
Updating from 41223... to 13134...
Fast-forward
$ git reset --hard ORIG_HEAD       (4)

1. Try to update from the upstream resulted in a lot of conflicts; you were not ready to spend a lot of time merging right now, so you decide to do that later.
2. "pull" has not made merge commit, so "git reset --hard" which is a synonym for "git reset --hard HEAD" clears the mess from the index file and the working tree.
3. Merge a topic branch into the current branch, which resulted in a fast-forward.
4. But you decided that the topic branch is not ready for public consumption yet.
   "pull" or "merge" always leaves the original tip of the current branch in ORIG_HEAD, so resetting hard to it brings your index file and the working tree back to that state, and resets the tip of the branch to that commit.

See HEAD and ORIG_HEAD for more.

Reset the master branch:

git reset --hard origin/master