Update the diff program to use exit code 1 for non-matching diff.

src/compare/diff.rs

@@ -54,6 +54,13 @@ impl DiffResult {
             .iter()
             .any(|child| child.status == DiffStatus::Bad || child.has_bad_children())
     }
+
+    pub fn is_bad(&self) -> bool {
+        match self.status {
+            DiffStatus::Good => !self.has_bad_children(),
+            DiffStatus::Bad => true,
+        }
+    }
 }
 
 pub fn compare_document<'s>(