Check enforcer should comment on the size of blobs added by a PR

Merge committing a PR will add all of the new blobs in the history of the source branch to the target branch's history. We should guard against large blobs in the PR history by auto-commenting on the size of new blobs added by a PR:

> Merging this PR with a merge-commit will add 120 blobs to the repository with a total size of 50MB
> Merging this PR with a squash-commit will add 10 blobs to the repository with a total size of 240KB

This will help identify large files that were added to a branch, but removed before the branch was submitted for PR. Because of git's default clone / fetch behavior, these deleted files will still affect the fetch size of the repository if the commit is merged.