At work, we recently updated from a massively old version of git (1.7.10)
to 2.18. There are a few code bases that use subtrees, and they seem to
have completely broke when trying to merge in updates.

I have confirmed that it works correctly in 1.7.10.  The 2.18 behavior is
clearly incorrect.

git init
echo init > test
git add test
git commit -m init

git remote add tig https://github.com/jonas/tig.git
git fetch tig
git merge -s ours --no-commit --allow-unrelated-histories tig-2.3.0
git read-tree --prefix=src/ -u tig-2.3.0
git commit -m "Get upstream tig-2.3.0"
# Notice how the history are merged, and that the source from the upstream
repo is in src

echo update > test
git commit -a -m "test"

git merge -s subtree tig-2.4.0
# Boom, in 2.18 instead of merging into the subtree, it just deletes
everything in the repository, which is clearly the wrong behavior.