Handle markers properly

[jackkamm]

PR https://github.com/jackkamm/undo-propose-el/pull/21 by @takaxp fixes a problem with org-clock-marker, but this problem is a symptom of a deeper problem, that we don't currently handle markers in the undo-propose buffers.

References

• Elisp manual: Overview of markers
• Elisp manual: Undo, see the entry on the marker element
• primitive-undo in simple.el, in particular these lines which handle marker adjustments.

Additional Notes

There are entries in the undo-list which adjust marker positions. When in the undo-propose buffer, (eq (marker-buffer m) (current-buffer)) is false so these entries are skipped.

One possible way to handle markers would be to copy the markers over to the undo-propose buffer, and edit entries in the undo-list to use the new markers.