fix: handle cargo milestone update for PR from GitHub merge queue

Cargo has migrated to GitHub merge queue:
rust-lang/cargo#14718

The regex doesn't work for merge commit made by GitHub merge queue.
This PR adds extra regex pattern to capture that.

This regex has been proved working locally.
See <ehuss/cargo-new-release#2>

src/handlers/milestone_prs.rs

@@ -145,24 +145,34 @@ async fn milestone_cargo(
     // <https://docs.github.com/en/rest/commits/commits?apiVersion=2022-11-28#list-pull-requests-associated-with-a-commit>,
     // but it is a little awkward to use, only works on the default branch,
     // and this is a bit simpler/faster. However, it is sensitive to the
-    // specific messages generated by bors, and won't catch things merged
-    // without bors.
-    let merge_re = Regex::new("(?:Auto merge of|Merge pull request) #([0-9]+)").unwrap();
-
-    let pr_nums = commits.iter().filter_map(|commit| {
-        log::info!(
-            "getting PR number for cargo commit {} (len={})",
-            commit.sha,
-            commit.commit.message.len()
-        );
-        merge_re.captures(&commit.commit.message).map(|cap| {
-            cap.get(1)
-                .unwrap()
-                .as_str()
-                .parse::<u64>()
-                .expect("digits only")
-        })
-    });
+    // specific messages generated by bors or GitHub merge queue, and won't
+    // catch things merged beyond them.
+    let merge_re =
+        Regex::new(r"(?:Auto merge of|Merge pull request) #([0-9]+)|\(#([0-9]+)\)$").unwrap();
+
+    let pr_nums = commits
+        .iter()
+        .filter(|commit|
+            // Assumptions:
+            // * A merge commit always has two parent commits.
+            // * Cargo's PR never got merged as fast-forward / rebase / squash merge.
+            commit.parents.len() == 2)
+        .filter_map(|commit| {
+            log::info!(
+                "getting PR number for cargo commit {} (len={})",
+                commit.sha,
+                commit.commit.message.len()
+            );
+
+            merge_re.captures(&commit.commit.message).map(|cap| {
+                cap.get(1)
+                    .or_else(|| cap.get(2))
+                    .unwrap()
+                    .as_str()
+                    .parse::<u64>()
+                    .expect("digits only")
+            })
+        });
     let milestone = cargo_repo
         .get_or_create_milestone(gh, release_version, "closed")
         .await?;