v5. Fixed a few oversights, but others undoubtedly remain.

If we're serious about preserving a message's original white space, then
various details related to filling and indenting still need hammering
out. And if that ultimately involves tampering with the two existing
fill functions (and such a prospect proves sufficiently unpopular), we
could always try adding dedicated variants that preserve original
spacing as their thing. It's also possible that such additions may end
up needing companions to unfill in their particular style.

But progress on these and other fronts will have to wait (unless someone
else wants to have a go) because this feature remains among ERC's lowest
priorities, ATM (IMO).

(Also, the undo situation is yet unexplored.)