This question is similar to this one, but more specific.
I have a project with two branches (staging and beta).
I develop
I encountered the same problem and solved it by
git checkout -f branch
and its specification is rather clear.
-f, --force
When switching branches, proceed even if the index or the working tree differs from HEAD. This is used to throw away local changes.
When checking out paths from the index, do not fail upon unmerged entries; instead, unmerged entries are ignored.