list-targets: Support sparse checkout

[suzuki-shunsuke]

Feature Overview

Enable list-targets to work without checking out all tfaction.yaml.

Why is the feature needed?

Currently, list-targets needs all tfaction.yaml to list changed directories.
But if the repository is huge, it takes a long time to checkout the whole repository.
In that case, sparse checkout is useful but list-targets doesn't work well.

Example Code

tfaction-root.yaml

list-targets:
  sparse-checkout: true

Note

1. Create a file .tfaction/services.txt at the repository root, which is a list of services. Using this file, list-targets can work without tfaction.yaml
2. Create or update .tfaction/services.txt by CI