Do not try to update head of current branch

src/git.ts

@@ -69,7 +69,7 @@ export async function getChangesSinceMergeBase(
     let lastCommitCount = await getCommitCount()
     let depth = Math.max(lastCommitCount * 2, initialFetchDepth)
     while (!(await hasMergeBase())) {
-      await exec('git', ['fetch', `--depth=${depth}`, 'origin', `${baseRef}:${baseRef}`, `${ref}:${ref}`])
+      await exec('git', ['fetch', `--depth=${depth}`, 'origin', `${baseRef}:${baseRef}`, `${ref}`])
       const commitCount = await getCommitCount()
       if (commitCount === lastCommitCount) {
         core.info('No more commits were fetched')