The retreve CI failures script can fail because of wrong backout identification

[marco-c]

https://hg.mozilla.org/integration/autoland/rev/e7df645db6e6 backed out https://hg.mozilla.org/integration/autoland/rev/81136daceeec, but the backout information is missing. This leads to a failure in the script:

Traceback (most recent call last):
  File "/usr/local/bin/bugbug-ci-failures-retriever", line 8, in <module>
    sys.exit(main())
             ^^^^^^
  File "/usr/local/lib/python3.12/site-packages/scripts/retrieve_ci_failures.py", line 396, in main
    raise e
  File "/usr/local/lib/python3.12/site-packages/scripts/retrieve_ci_failures.py", line 394, in main
    _ = task.result()
        ^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/concurrent/futures/_base.py", line 449, in result
    return self.__get_result()
           ^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/concurrent/futures/_base.py", line 401, in __get_result
    raise self._exception
  File "/usr/local/lib/python3.12/concurrent/futures/thread.py", line 58, in run
    result = self.fn(*self.args, **self.kwargs)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/site-packages/scripts/retrieve_ci_failures.py", line 309, in generate_diffs
    diff = diff_failure_vs_fix(
           ^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/site-packages/scripts/retrieve_ci_failures.py", line 247, in diff_failure_vs_fix
    ["git", "-C", repo, "read-tree", f"{failure_commits[0]}^"],
                                        ~~~~~~~~~~~~~~~^^^
IndexError: list index out of range