Try again to preserve undo history

[leungbk]

Closes #15.

[leungbk]

It seems that undo-amalgamate-change-group is not available prior to Emacs 26. Should we just paste in the source? Or should we simply drop support for 25?

[raxod502]

Emacs 25 is still used enough that I think I'd prefer to continue supporting it. In particular it's shipped with the second-to-latest Ubuntu LTS which most people are probably still on who run Ubuntu LTS.

Pasting in the definition is fine, here is an example of how to do that.

[leungbk]

I closed the PR since it didn't seem to do anything to alleviate the problem of large entries in buffer-undo-list.

Tha markers generated within apheleia--apply-rcs-patch seem to be accounting for much of the size of each entry within buffer-undo-list. But from a glance at the code, the markers look pretty important, so I'm not sure how the space usage of this function can be cut down.

[raxod502]

Oh, huh. I didn't realize markers got recorded in the undo history. I guess that makes sense though. Do we clean them up properly? I don't see anything for that in the code. Perhaps they are accumulating over time and the undo history is getting exponentially bigger because of all the markers getting moved around.