Improve squash_commit_message to ref original MR (#17)

Author: janedbal

src/MergeRequestAcceptor.ts

@@ -228,7 +228,7 @@ export const acceptMergeRequest = async (gitlabApi: GitlabApi, mergeRequest: Mer
 			should_remove_source_branch: true,
 			merge_when_pipeline_succeeds: true,
 			sha: mergeRequestInfo.diff_refs.head_sha,
-			squash_commit_message: `${mergeRequest.title} (#${mergeRequest.iid})`,
+			squash_commit_message: `${mergeRequest.title} (!${mergeRequest.iid})`,
 		});
 
 		if (response.status === 405) { // cannot be merged