Helper to merge / append to history records

[mjsir911]

Problem Statement

We've run in to the situation a few times where multiple .save()s need to be called in short succession / a single "change" and it would be convenient to be able to specify that these multiple saves should be recorded as a single history record.

We run into this same problem when tracking m2m changes too since those changes set off a signal that trigger creation of additional history records ( I think there's discussion about this in #399 )

Describe the solution you'd like

A few potential interfaces:

• something like skip_history_when_saving but append_history_when_saving or similar
• HistoryRecord.merge_with_previous() / HistoryRecord.merge_into_and_delete(record)
• .history.merge_till(record)
• some sort of context manager?

For all of these ways to go about this, what they would functionally do is just delete earlier records & potentially copy over date/user/change reason information.

One unanswered question is how multiple change reasons / change users on a record would work. Probably just have to arbitrarily choose to keep one (I would prefer to keep the change reason on the earliest history record)

Describe alternatives you've considered

Theoretically a workaround is temporarily not recording history then doing a final .save() with history recording enabled, or deleting each created record on a new .save(), or appending changes to an existing history record with .latest(). I'de just like to formalize a mechanism to do this & not have to reinvent ways to achieve this every time.