Implement post-commit review request creation.

Our webapi already has the plumbing necessary to create post-commit reviews, and
in a few previous changes I added the scmtools/hostingsvcs infrastructure for
fetching the necessary data. This change bridges the gap by making
update_from_commit_id do the right thing if said commit_id refers to an already
committed revision on a repository that supports get_change.

- Ran unit tests.
- Used this with my UI.