conflicts: don't panic when a conflict marker is missing removes

Closes #2611

CHANGELOG.md

@@ -22,6 +22,9 @@ to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ### Fixed bugs
 
+ * Fixed panic when parsing invalid conflict markers of a particular form.
+   ([#2611](https://github.com/martinvonz/jj/pull/2611))
+
 ## [0.21.0] - 2024-09-04
 
 ### Breaking changes

lib/src/conflicts.rs

@@ -456,7 +456,7 @@ fn parse_conflict_hunk(input: &[u8]) -> Merge<BString> {
                     removes.last_mut().unwrap().extend_from_slice(rest);
                     adds.last_mut().unwrap().extend_from_slice(rest);
                 } else {
-                    // Doesn't look like a conflict
+                    // Doesn't look like a valid conflict
                     return Merge::resolved(BString::new(vec![]));
                 }
             }
@@ -467,13 +467,18 @@ fn parse_conflict_hunk(input: &[u8]) -> Merge<BString> {
                 adds.last_mut().unwrap().extend_from_slice(line);
             }
             State::Unknown => {
-                // Doesn't look like a conflict
+                // Doesn't look like a valid conflict
                 return Merge::resolved(BString::new(vec![]));
             }
         }
     }
 
-    Merge::from_removes_adds(removes, adds)
+    if adds.len() == removes.len() + 1 {
+        Merge::from_removes_adds(removes, adds)
+    } else {
+        // Doesn't look like a valid conflict
+        Merge::resolved(BString::new(vec![]))
+    }
 }
 
 /// Parses conflict markers in `content` and returns an updated version of

lib/tests/test_conflicts.rs

@@ -750,8 +750,6 @@ fn test_parse_conflict_wrong_arity() {
 }
 
 #[test]
-// TODO: Should *not* panic
-#[should_panic]
 fn test_parse_conflict_malformed_missing_removes() {
     // Right number of adds but missing removes
     assert_eq!(