Function change_summary 

pub fn change_summary(
    doc: &mut AutoCommit,
    from: &[ChangeHash],
    to: &[ChangeHash],
) -> String

Describe, in one short phrase, what moving doc from from to to does: "+12 -3 chars", "+1 item", "2 fields".

The public face of the same producer the history rows are built from, so an Undo control and the timeline describe an action identically rather than each inventing wording. Like every summary here it never guesses: a change whose effect cannot be described comes back as NO_VISIBLE_CHANGE.

&mut AutoCommit because reading a diff flushes the open transaction; the document is not otherwise modified.