All of our ReviewBot Tools only run when a diff has been updated, and ignores

other review request updates (for example, if the summary or description is

updated).
There are some tools, however, that might want to analyze the review request

fields rather than the diff. For example, a Tool could be written to enforce

a certain style guide in the Description and Testing Done fields, or do

spell checking.
This change lets Tools opt-in to running when a review request has changed,

even if the diff wasn't updated.

I wrote a very simple ReviewBot Tool that ensures that the "Testing Done"

field has some content in it, and it successfully ran when I updated

only the Summary. It also still worked when I posted a review request

with a diff.