Function isoformat_roundtrip 

pub fn isoformat_roundtrip(stamp: &str) -> String

Python datetime.fromisoformat(stamp).isoformat() round trip of a git %cI stamp: verbatim for the ±HH:MM form git emits; a trailing Z re-serializes as +00:00, a colonless ±HHMM gains its colon, ±HH becomes ±HH:00, and a space separator becomes T.