(refill-adjust-ignorable-overlay): New function.
(refill-pre-command-function): New function.
(refill-mode): Add `refill-pre-command-function' to `pre-command-hook'.
Initialize/cleanup `refill-ignorable-overlay'.
(refill-post-command-function): Don't reset refill-doit in the case
where a self-insertion command doesn't case a refill. Use
`refill-fill-paragraph-at', getting position from `refill-doit'.
(refill-after-change-function): Set `refill-doit' to END.
(refill-fill-paragraph-at): New function, mostly from old
refill-fill-paragraph. Use `refill-ignorable-overlay' to fill only the
paragraph's tail if possible. Update `refill-ignorable-overlay'. Don't
leave point inside the fill-prefix.
(refill-fill-paragraph): Use `refill-fill-paragraph-at'.
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