Rules integration for Diff module

#114699: Diff module should support Token has a similar agenda and so does #1365750: Generalize API and Integrate with core field types.
This might be a duplicate of those, see also #996696: Support revisions in Entity API.

Examples:
* When node is updated, send node author an email with revision diff.
* When user modifies cart, show a system message with a diff
* When node is changed, if the new revision removes # lines, ...
* When a node is changed, if the diff is empty, ...

Actions:
* Calculate the difference between two things

Conditions:
* Diff contains # of +/-'s. ie: #1671484: Show number of lines changed on revisions page