Improve Undo Last Discard

[wadethestealth]

Summary

Improve the intelligence of the undo-last-discard command. This would essentially allow for undo last discard to remember or know of the previous markers/hunks it removed, so that undoing would allow for picking which changes of the undo you actually want back.

Motivation

A user (@rdpoor) accidentally discarded all changes and used the undo command, however when the undo command undoes changes with file markers, it marks the whole file as in conflict rather than just the lines that are different.

Additional context

About around where original discussion begins in slack