fix bug of commit is not an ancestor of the current commit

[xdays]

Checklist

• All new jobs, commands, executors, parameters have descriptions
• Examples have been added for any significant new features
• README has been updated, if necessary

Motivation, issues

Fix the bug of commit is not an ancestor of the current commit

Description

if we push a new branch and the commit id of previous job is not an ancestor of the current commit then RETURN_CODE is 1, when the script really find an ancestor of the current commit
git merge-base --is-ancestor $COMMIT_FROM_JOB_NUM $CIRCLE_SHA1 || { RETURN_CODE=$?; }, the git command return 0, but { RETURN_CODE=$?; } will not executed due to ||

[xdays]

I will close this PR as there's already one #36 for fixing this bug.