This is needed in order to support the ability to check out the repo as
it would appear after the pull request is merged into the base branch.

This flag can be set to either branch (default) or merge. If set to
branch, we will check out the head branch of the pull request (the
source). If set to merge, we will check out the base branch of the pull
request (the destination) and then attempt to perform a git merge of the
pull request branch. This simulates what will happen if the pull request
is merged.

This allows us to perform terraform commands on what the state of the
repo will be *after* the pull request is merged. This is useful if users
are often opening up pull requests from branches that aren't up to date
with master. With the branch strategy, the terraform plan might be
deleting resources that have been created in the master branch but with
the merge strategy, we merge the branch's changes into the master branch
and so don't have this problem.

Also disable test linters since we already run tests.