Implement the change from the previous commit for GitLab and Bitbucket
Server/Cloud. Test the change for GitHub.
If a file is moved, we want to run plan in the directory it is moved to
*and* the directory it was moved from.
Introduce new `mergeable` requirement, in similar vein to the `approved`
requirement. Ran `make go-generate` to update mocks accordingly.
This addresses issue #43.