Fix getting correct commit from multiple referenced PR (#33411)

When a PR is referenced by other PRs, our dev tool for getting the correct
commit lists the latest commit when looking for the commmit sha but we should get the oldest.

(cherry picked from commit 5b104a9)

dev/airflow-github

@@ -83,9 +83,10 @@ def get_issue_type(issue):
 def get_commit_in_main_associated_with_pr(repo: git.Repo, issue: Issue) -> str | None:
     """For a PR, find the associated merged commit & return its SHA"""
     if issue.pull_request:
-        commit = repo.git.log(f"--grep=#{issue.number}", "origin/main", "--format=%H")
+        commit = repo.git.log("--reverse", f"--grep=#{issue.number}", "origin/main", "--format=%H")
         if commit:
-            return commit
+            # We only want the oldest commit that referenced this PR number
+            return commit.splitlines()[0]
         else:
             pr: PullRequest = issue.as_pull_request()
             if pr.is_merged():