Function parse_diff_summary 

pub fn parse_diff_summary(output: &str) -> Vec<FileChange>

Parse jj diff --summary output into structured FileChange values.

jj produces lines like:

M path/to/file.rs
A new_file.rs
D removed.rs
R old/path.rs -> new/path.rs
C from.rs -> to.rs

Unknown status letters are skipped. Blank lines are skipped.