If `source_branch` option is passed, and target branch cannot be found,
`Repository#update_branch_with_hooks` would try to create a new branch
from `source_branch`.
This way, we could make changes in the new branch while only firing
the hooks once for the changes. Previously, we can only create a new
branch first then make changes to the new branch, firing hooks twice.
This behaviour is bad for CI.
Fixes#7237
