Run Atlantis plan in directories modified by pre workflow run

[nitrocode]

Discussed in #2690

Related to #2538

Currently Atlantis finds changes in files that were committed to the PR.

Here is an example in the github vcs client.

atlantis/server/events/vcs/github_client.go

Lines 104 to 120 in 89e035c

	func (g *GithubClient) GetModifiedFiles(repo models.Repo, pull models.PullRequest) ([]string, error) {
	var files []string
	nextPage := 0
	listloop:
	for {
	opts := github.ListOptions{
	PerPage: 300,
	}
	if nextPage != 0 {
	opts.Page = nextPage
	}
	// GitHub has started to return 404's sometimes. They've got some
	// eventual consistency issues going on so we're just going to attempt
	// up to 5 times for each page with exponential backoff.
	maxAttempts := 5
	attemptDelay := 0 * time.Second

atlantis/server/events/project_command_builder.go

Lines 213 to 215 in 89e035c

	func (p *DefaultProjectCommandBuilder) buildPlanAllCommands(ctx *command.Context, commentFlags []string, verbose bool) ([]command.ProjectContext, error) {
	// We'll need the list of modified files.
	modifiedFiles, err := p.VCSClient.GetModifiedFiles(ctx.Pull.BaseRepo, ctx.Pull)

It would be nice to use a pre_workflow_run to modify terraform in other directories and have Atlantis pick it up automatically.

We could do this by using the native git client. We already have context of the prs base repo and base branch and pr branch. Once we run the pre workflow run, we should not hit the vcs api and instead just run the following.

git diff --name-only SHA1 SHA2