Auto merge of #13581 - LuuuXXX:issue-12692, r=weihanglo

fix(ci): bump check error when PR is behind master

### What does this PR try to resolve?

Fixes: #12692

crates/xtask-bump-check/src/xtask.rs

@@ -333,10 +333,8 @@ fn changed<'r, 'ws>(
             (relative_pkg_root, pkg)
         })
         .collect::<Vec<_>>();
-    let base_tree = base_commit.as_object().peel_to_tree()?;
-    let head_tree = head.as_object().peel_to_tree()?;
-    let diff = repo.diff_tree_to_tree(Some(&base_tree), Some(&head_tree), Default::default())?;
 
+    let diff = symmetric_diff(repo, base_commit, head)?;
     let mut changed_members = HashMap::new();
 
     for delta in diff.deltas() {
@@ -354,6 +352,22 @@ fn changed<'r, 'ws>(
     Ok(changed_members)
 }
 
+/// Using a "symmetric difference" between base and head.
+fn symmetric_diff<'a>(
+    repo: &'a git2::Repository,
+    base: &'a git2::Commit<'a>,
+    head: &'a git2::Commit<'a>,
+) -> CargoResult<git2::Diff<'a>> {
+    let ancestor_oid = repo.merge_base(base.id(), head.id())?;
+    let ancestor_commit = repo.find_commit(ancestor_oid)?;
+    let ancestor_tree = ancestor_commit.as_object().peel_to_tree()?;
+    let head_tree = head.as_object().peel_to_tree()?;
+    let diff =
+        repo.diff_tree_to_tree(Some(&ancestor_tree), Some(&head_tree), Default::default())?;
+    tracing::info!(merge_base = %ancestor_commit.id(), base = %base.id(), head = %head.id(), "git diff base...head");
+    Ok(diff)
+}
+
 /// Compares version against published crates on crates.io.
 ///
 /// Assumption: We always release a version larger than all existing versions.