First, unstage the merge conflicts using the following command:
$ git reset HEAD .
Then save the conflicted merge
$ git stash
Now return to master
$ git checkout master
In order to pull latest changes:
$ git fetch upstream; git merge upstream/master
In order to correct my new branch:
$ git checkout new-branch; git rebase master
In order to apply the correct stashed changes (now 2nd on the stack):
$ git stash apply stash@{1}