Function changelog 

pub fn changelog(key: &str, changes: &[Change], ctx: &Context) -> String

What changed on an issue, one line per field.

The unit is a field, not an event: “who set the status to Closed” is the question, and an event that touched three fields answers it three times over only if it is split. The event columns repeat as a result, which is the price of every line being readable on its own.

An event that changed nothing a caller can see — a transport detail, a re-index — still gets a line, with - for the field. Dropping it would make the history look like it has gaps.