Previously `start_sha` was intercepted on the frontend to create the correct branch in a separate API call. Now that the commits API supports the `start_sha` parameter directly this workaround is not needed anymore.
**Why?** The branch HEAD could be changed since the IDE was opened. This leads to user's unintentionally creating commits that overwrite other changes. https://gitlab.com/gitlab-org/gitlab-ce/issues/59023
