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