Add module that creates a permanent revision from a significantly modified autosave

[jsnajdr]

When the editor does an autosave repeatedly, create a permanent revision from the old
autosave if the new one is significantly different. Makes losing edited content less
likely.

Adapted from a WP.com patch (D30389-code), converted the code to a Jetpack module and
added (optional) recording of StatsD events using the new jetpack_stats_statsd action
(that will be hooked into on WP.com).

Fixes an issue reported in Calypso repo: Automattic/wp-calypso#20265

Testing instructions:

(adapted from D30389-code)
This module creates a permanent revision if new autosave's content length is more than 250 characters smaller or bigger than the old one's.

• edit a draft or published post in various editors (Classic or Block)
• add one character and wait for autosave. Check that permanent revision was not created. (best done with revisions page opened in a second browser tab) You see a revision for the post content before you started editing, and an autosave revision with the one new char.
• add 250+ characters and wait for autosave. Check that permanent revision was created from the previous autosave (+1 char edit)
• start editing again from a fully saved state. Add 250+ characters and wait for autosave. Check that permanent revision was not created: the content before the autosave already has a revision and the one created during autosave would be an identical one.

Proposed changelog entry for your changes:

• Writing: more reliable autosaves that save a permanent revision before large changes

[jetpackbot]

Thank you for the great PR description!

When this PR is ready for review, please apply the [Status] Needs Review label. If you are an a11n, please have someone from your team review the code if possible. The Jetpack team will also review this PR and merge it to be included in the next Jetpack release.

Scheduled Jetpack release: August 6, 2019.
Scheduled code freeze: July 30, 2019

Generated by 🚫 dangerJS against ed96d57

[jeherve]

Since you are adding a brand new file, could you add it to the PHPCS whitelist?
https://github.com/Automattic/jetpack/blob/5a2bb2b2423a42a59ced3674f30339605fbc7fd5/bin/phpcs-whitelist.js

That will allow you to get some recommendations on commit, and to fix any PHPCS issues on the file.

[jsnajdr]

@jeherve The PHPCS whitelist add is already in 65edf25

[donpark]

LGTM

Works like WPCOM version of the patch. BTW, I'm not a Jetpack code owner so you'll need ✅ from @jeherve .

[jeherve]

@jeherve The PHPCS whitelist add is already in 65edf25

Excellent! Do you think you could now fix the issues that pop up? You should see them in your editor, or when running vendor/bin/phpcs -w modules/autosave-revisions.php.

[jsnajdr]

Do you think you could now fix the issues that pop up?

Yes! 🙂 This PR will be most likely closed and we'll ship the code to Atomic via a different route. Let's keep in open while other options are evaluated.

[jsnajdr]

We shipped this to Atomic as part of wpcomsh instead, can be closed. Thanks everyone for cooperation.