webhooks: include old revision in MR update events

## What does this MR do?

Includes the old revision for an MR in webhooks.

## Are there points in the code the reviewer needs to double check?

The docs do not give an example `update` hook; should I add one?

## Why was this MR needed?

In order to keep web hook listeners stateless, it helps to include the previous revision of the MR in the hook so that it does not need to remember the last revision (which may also be wrong in case of missed or out-of-order events due to previous errors).

## What are the relevant issue numbers?

N/A

## Screenshots (if relevant)

N/A

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [ ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [x] API support added
- Tests
  - [x] Added for this feature/bug
  - [x] All builds are passing
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !5535