feat(source): support cherry-picking commits for git sources

- add source.cherry_pick (and cherry-pick alias) to package spec parsing/serialization
- apply listed cherry-picks in order after git checkout
- reject cherry_pick on non-git sources with explicit error
- include cherry_pick in interactive spec TOML output
- add parsing/behavior tests and update Source test fixtures

src/builder/mod.rs

@@ -267,6 +267,7 @@ mod tests {
                 extract_dir: "src".into(),
                 patches: Vec::new(),
                 post_extract: Vec::new(),
+                cherry_pick: Vec::new(),
             }],
             build: Build {
                 build_type: BuildType::Custom,