This change adds a signal for when a diffset has been uploaded to a review

request. Ideally we would emit this signal in one central place for all

diffsets, but our codebase creates diffsets in a few different ways, with each

having different points at which the full state of the diffset is initialized

and ready. The three types of diffset creation are:

- Creating traditional diffsets that are tied to a single commit

- Creating commit series diffsets that support multiple commits

- Creating diffsets from existing commits
It's tricky because after a diffset is created, some attributes and related

state such as the review request draft or file diffs are not guaranteed to

be ready. And each of these three types of diffset creation have different

points at which the related objects and state are all fully initialized.

We don't want to emit the signal before all of these are ready because

listeners might try to access some state that may not exist yet. So, we take

special care to emit the signal during each type of diffset creation.

Ran unit tests.
Tested with an upcoming assign reviewers by regex extension, which

  listens to the signal.
Tested uploading diffs that get created in the three types of ways

  mentioned in the description, saw that the signal was emitted each time.

  These are the different ways that I uploaded diffs:
Using rbt post, also with a given revision, revision range, and

--diff-filename.
Dragging and dropping a diff into the new review request page.
Using the "Update diff" button on a review request page.
Choosing an existing commit on the new review request page.