generate a diff summary

Problem/Motivation

We needed to remove the autogenerated diff summary at:
#2880936: Remove runtime generated summary: Diff overhead on revisions gets exponential

There were two problems: It was slow and the output (field names) didn't really help users.
That doesn't change the situation: The revision log should display meaningful information if the user doesn't provide any text.

Proposed resolution

Instead of field names, the effective content changes (at least to text) would be much more helpful.

To solve the performance problem, we should generate a message on save.
(That message would need to be generated in bulk for existing entity revisions.)

Best would be to have a separate field for the generated summary. For that we though need to alter the schema of revision tables...
Alternatively we can write the diff into the message log. To identify these messages so we can regenerate them if our text format changes) we could use some UTF-8 characters like the delta to indicate change (and hope that people don't use it in their revision logs :-)).
Note that some people would like to see both message log and summary. #2921962: Display log message in addition to changed fields

Remaining tasks

User interface changes

API changes

Data model changes