Stop polluting registers when (re)generating TOC

[SidOfc]

I noticed some strange behavior when moving old changes from the README to the CHANGELOG.
After cutting the notes with x, saving the file, and switching to the CHANGELOG, pasting with p resulted in pasting the actual table of contents of the README instead.

This happens whenever the table of contents is (re)generated:

This should not happen and must be fixed. I think this can be achieved by storing the old registers before updating the buffer and then copying the old contents back into the registers that were overwritten.