repository.writeFile could overwrite the changes

The file changes are overwritten when repository.writeFile is called to save the file which has been updated by the others.

Start at the repository.writeFile entry point and trace how it saves a file through the GitHub API. Reproduce the case where another user's changes exist, then identify the expected conflict behavior and add coverage for it; done means those changes are no longer silently overwritten.

索引モデルが issue の本文から書いたものです。