message: Handle ambiguous refname warning.
diff: @@ -398,7 +398,7 @@ export async function getUpdateMessage() {
  */
  export async function findMergeBase(headRef, baseRef) {
  const result = await execGitCommand(`git merge-base --all ${headRef} ${baseRef}`);
  - const mergeBases = result.split(EOL).filter(Boolean);
  + const mergeBases = result.split(EOL).filter(line => line && !line.startsWith('warning: '));
  if (mergeBases.length === 0) return undefined;
  if (mergeBases.length === 1) return mergeBases[0];
  