check PR changed files count directly from get pull request endpoint

[dblinkhorn]

Implements a fix for the checking of the files changed count for a PR. Now the check will be against the git pull request endpoint changed_files property rather than the list of files returned from the list pull request files endpoint.

[palantirtech]

Thanks for your interest in palantir/policy-bot, @dblinkhorn! Before we can accept your pull request, you need to sign our contributor license agreement - just visit https://cla.palantir.com/ and follow the instructions. Once you sign, I'll automatically update this pull request.

[bluekeyes]

Thanks for submitting this! I think we can simplify the implementation by taking advantage of the fact that we already load the pull request object when creating the pull.Context

[bluekeyes]

I shouldn't be necessary to add a new method to get the changed files from the pull request object because we load the pull request as part of creating the GitHubContext object. Instead, I think we need to extend the stored object to include this field:

1. Modify the v4PullRequestStruct to include a new ChangedFiles field
2. Update Locator.IsComplete to return false if the ChangedFiles field is missing or empty
3. Update Locator.toV4 to se the new ChangedFiles field from the github.PullRequest object in the Locator

After that, you should be able to use ghc.pr.ChangedFiles to reference this in the ChangedFiles method