Function ref_update_line 

pub fn ref_update_line(
    old: Option<&Hash>,
    new: &Hash,
    src: &str,
    dst: &str,
    forced: bool,
) -> String

A single git-style ref-update summary line, as printed under the To <url> / From <url> header of a push or fetch. old is the previous value of the destination ref (None = the ref did not exist), new the value just written; src -> dst is the refspec mapping.

Shapes match git’s transport.c for the single-ref case:

• new ref: * [new branch] <src> -> <dst>
• forced: + <old>...<new> <src> -> <dst> (forced update)
• fast-fwd: <old>..<new> <src> -> <dst>

Object ids are mkit BLAKE3 prefixes rather than git SHA-1 (documented divergence); everything else is byte-shaped like git.