Fix a minor issue in the update-release-branch.py script that performs a
call to `sorted` but doesn't use the output. Since `sorted` does not
operate in place, the call is currently useless. As a result, the function
`get_pr_for_commit` does not currently work as exected. I.e. it is
expected to return the "first" (i.e. lowest PR number), but actually it
returns the first in the list provided by GitHub.
