ep: Fix header for target patches (#57598)

Previously, we were always copying the _first_ header of the source
patch, which is wrong for 99% of time.

Release Notes:

- N/A

crates/edit_prediction_cli/src/split_commit.rs

@@ -394,7 +394,12 @@ pub fn generate_evaluation_example_from_ordered_commit(
 pub fn split_ordered_commit(commit: &str, split_pos: usize) -> (String, String) {
     let patch = Patch::parse_unified_diff(commit);
     let source_edits: BTreeSet<usize> = (0..split_pos).collect();
-    let (source, target) = extract_edits(&patch, &source_edits);
+    let (source, mut target) = extract_edits(&patch, &source_edits);
+    if !target.hunks.is_empty() {
+        if let Some(header) = header_for_edit(&patch, split_pos) {
+            target.header = header;
+        }
+    }
 
     let mut source_str = source.to_string();
     let target_str = target.to_string();
@@ -419,6 +424,47 @@ pub fn split_ordered_commit(commit: &str, split_pos: usize) -> (String, String)
     (source_str, target_str)
 }
 
+fn header_for_edit(patch: &Patch, edit_index: usize) -> Option<String> {
+    let edit_index = edit_index.try_into().ok()?;
+    let edit_location = locate_edited_line(patch, edit_index)?;
+    header_for_hunk(patch, edit_location.hunk_index)
+}
+
+fn header_for_hunk(patch: &Patch, hunk_index: usize) -> Option<String> {
+    for hunk in patch.hunks.get(..hunk_index)?.iter().rev() {
+        let mut header_lines = Vec::new();
+        for line in hunk.lines.iter().rev() {
+            let PatchLine::Garbage(line) = line else {
+                break;
+            };
+            if line.trim().is_empty() && header_lines.is_empty() {
+                continue;
+            }
+            if !line.starts_with("//") {
+                break;
+            }
+            header_lines.push(line.as_str());
+        }
+        if !header_lines.is_empty() {
+            return Some(render_reversed_header_lines(header_lines));
+        }
+    }
+
+    let header_lines = patch
+        .header
+        .lines()
+        .rev()
+        .skip_while(|line| line.trim().is_empty())
+        .take_while(|line| line.starts_with("//"))
+        .collect::<Vec<_>>();
+    (!header_lines.is_empty()).then(|| render_reversed_header_lines(header_lines))
+}
+
+fn render_reversed_header_lines(mut lines: Vec<&str>) -> String {
+    lines.reverse();
+    lines.join("\n") + "\n"
+}
+
 /// Calculate the weight for a split position based on the character at that position.
 ///
 /// Higher weights indicate more natural pause points (e.g., after punctuation,
@@ -1179,6 +1225,55 @@ mod tests {
         assert_eq!(tgt_patch.stats().added, 1);
     }
 
+    #[test]
+    fn test_split_ordered_commit_target_header_continues_current_group() {
+        let commit = r#"////////////////////////////////////////////////////////////////////////////////
+// Update dependency version
+////////////////////////////////////////////////////////////////////////////////
+--- a/go.mod
++++ b/go.mod
+@@ -1,3 +1,3 @@
+ require (
+-	gopkg.in/yaml.v3 v3.0.0 // indirect
++	gopkg.in/yaml.v3 v3.0.1 // indirect
+ )
+diff --git a/go.sum b/go.sum
+index f71a068..b8cc3c2 100644
+////////////////////////////////////////////////////////////////////////////////
+// Update go.sum checksums
+////////////////////////////////////////////////////////////////////////////////
+--- a/go.sum
++++ b/go.sum
+@@ -1,3 +1,5 @@
+ gopkg.in/yaml.v3 v3.0.0 h1:old
+ gopkg.in/yaml.v3 v3.0.0/go.mod h1:oldmod
++gopkg.in/yaml.v3 v3.0.1 h1:new
++gopkg.in/yaml.v3 v3.0.1/go.mod h1:newmod
+diff --git a/lib/handler.go b/lib/handler.go
+index 1827a70..d9b3ed1 100644
+////////////////////////////////////////////////////////////////////////////////
+// Fix error wrapping
+////////////////////////////////////////////////////////////////////////////////
+--- a/lib/handler.go
++++ b/lib/handler.go
+@@ -1,3 +1,3 @@
+-	return fmt.Errorf("failed: %s", err)
++	return fmt.Errorf("failed: %w", err)
+"#;
+
+        let (_source, target) = split_ordered_commit(commit, 3);
+
+        assert!(
+            target.starts_with(
+                "////////////////////////////////////////////////////////////////////////////////\n// Update go.sum checksums\n////////////////////////////////////////////////////////////////////////////////\n"
+            ),
+            "target patch should continue with the active group header:\n{target}"
+        );
+        assert!(!target.starts_with(
+            "////////////////////////////////////////////////////////////////////////////////\n// Update dependency version\n////////////////////////////////////////////////////////////////////////////////\n"
+        ));
+    }
+
     #[test]
     fn test_generate_evaluation_example() {
         let commit = r#"commit abc123