One way of many...
This is one way which keeps the history looking like you'd expect.
Command is equivalent to git checkout MINE; git merge -s theirs HERS
.
git checkout MINE
git merge --no-commit -s ours HERS
git rm -rf .
git checkout HERS -- .
git commit
From http://www.seanius.net/blog/2011/02/git-merge-s-theirs/