[snippet-bot] post a new comment if snippets change on new commits

[thatfiredev]

It would be nice if snippet-bot could auto-refresh every time a new commit is pushed to a PR.

The reason for this is that we may sometimes forget to check "Refresh this comment" (or add the force-run label) and end up committing unintended snippet changes.

An example can be seen in googlemaps-samples/android-places-demos#802:

1. snippet-bot reported an untintentional deletion of 2 region tags.
2. Developer pushed a commit that fixed 1 (out of 2) of the region tags, but forgot to refresh the comment.
3. PR was merged with 1 unintentional snippet deletion.

This wouldn't happen if the bot had posted a new comment after the new commit was pushed to that branch.