# Migrate to the branch you want to merge into
git checkout <branch_name>
# Merge branch_b into branch you're in
git merge <branch_name>
# Force merge if the branches have unrelated histories. This means
# you must have previously gotten an error message like this
# 'fatal: refusing to merge unrelated histories' when you try to do
# normal merge with the command above.
git merge -X theirs <branch_name> --allow-unrelated-histories