refs: retry trivial resolution after merging targets

This helps resolve diverged refs by abandoning both sides:

    D  ref = [D]
    |\
    | C       C  ref = [B - D + C]
    | |       |
    B |     B |     B  ref = [B - D + A]
    |/      |/      |
    A       A       A       A  ref = [A - D + A]

lib/src/refs.rs

@@ -102,10 +102,16 @@ pub fn merge_ref_targets(
     ])
     .flatten()
     .simplify();
-    if !merge.is_resolved() {
+    // Suppose left = [A - C + B], base = [B], right = [A], the merge result is
+    // [A - C + A], which can now be trivially resolved.
+    if let Some(resolved) = merge.resolve_trivial() {
+        RefTarget::resolved(resolved.clone())
+    } else {
         merge_ref_targets_non_trivial(index, &mut merge);
+        // TODO: Maybe better to try resolve_trivial() again, but the result is
+        // unreliable since merge_ref_targets_non_trivial() is order dependent.
+        RefTarget::from_merge(merge)
     }
-    RefTarget::from_merge(merge)
 }
 
 pub fn merge_remote_refs(

lib/tests/test_refs.rs

@@ -363,6 +363,36 @@ fn test_merge_ref_targets() {
         target4
     );
 
+    // Existing conflict on left, right moves one side of conflict to the other
+    // side ("A - B + A" - type conflict)
+    assert_eq!(
+        merge_ref_targets(
+            index,
+            &RefTarget::from_legacy_form(
+                [commit5.id().clone()], // not an ancestor of commit3, 4
+                [commit3.id().clone(), commit4.id().clone()],
+            ),
+            &target4,
+            &target3,
+        ),
+        target3
+    );
+
+    // Existing conflict on right, left moves one side of conflict to the other
+    // side ("A - B + A" - type conflict)
+    assert_eq!(
+        merge_ref_targets(
+            index,
+            &target4,
+            &target3,
+            &RefTarget::from_legacy_form(
+                [commit5.id().clone()], // not an ancestor of commit3, 4
+                [commit3.id().clone(), commit4.id().clone()],
+            ),
+        ),
+        target4
+    );
+
     // Existing conflict on left, right makes unrelated update
     assert_eq!(
         merge_ref_targets(