Support running on [push]

[developit]

As requested on Twitter and in #6, it would be nice to allow running this action on push, in addition to the pull_request workflow. The action should comment on the push with size deltas from its parent commit.

[developit]

This may be fixable via #18, since Checks are supported for individual commits. Still need to figure out what a logical base would be for size comparison.

[developit]

I figured out a GraphQL query that can potentially be used to run on push but get the associated PR for a given push (payload properties):

{
  repository(name: "mitt", owner: "jaylinski") {
    object(expression: "e5c9e1505b83fe6e35d54aa5db62aa055921bea5") {
      ...on Commit {
        prs: associatedPullRequests(first: 1, orderBy:{field:UPDATED_AT,direction:DESC}) {
          nodes {
            number,
            resourcePath,
            isCrossRepository,
            baseRefName,
            mergeCommit: potentialMergeCommit { oid }
          }
        }
      }
    }
  }
}

[developit]

Potentially can be fixed by #35.

[andrewiggins]

I updated #35 and marked it ready for review