commit_graph/segments: panic with a more descriptive message on missing parent segment

Summary:
Context: When ordering commit graph segments, there's an assumption that any segment will only have a location associated with a parent changeset ID if there's a parent segment with that location's head. We panic when this assumption doesn't hold.

This diff: Make the panic message more descriptive to make debugging easier.

Reviewed By: gustavoavena

Differential Revision: D101337656

fbshipit-source-id: b5a21660976eb2ba745b56f6dde1dbbeb9b5249a

Author: YousefSalama

eden/mononoke/repo_attributes/commit_graph/commit_graph/src/segments.rs

@@ -713,7 +713,14 @@ impl<E: EdgeType> CommitGraphOps<E> {
                     }
                 }
             }
-            sorted_segments.push(segments.remove(&segment_head).unwrap());
+            if let Some(segment) = segments.remove(&segment_head) {
+                sorted_segments.push(segment);
+            } else {
+                panic!(
+                    "Segment head {} not found in segments {:?}",
+                    segment_head, segments
+                );
+            }
         }
         sorted_segments
     }