Fix a false negative in UselessAssignment with nested conditionals

[yujinakayama]

The cop can detects the following useless assignment:

if flag_a
  # This is useless ...
  foo = 1
else
  # ... since this will never run together with the assignment
  puts foo
end

This is done by checking if branches of the assignment and reference are exclusive.

However, previously it couldn't detect the following useless assignment:

if flag_a
  # This is useless ...
  foo = 1
else
  if flag_b
    # ... since this will never run together with the assignment
    puts foo
  end
end

Because the old logic worked correctly only when the both branches belong to a same conditional (i.e. paired if and else).

With the new logic it can detect such useless assignments by checking ancestor branches.

---

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• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
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• Squashed related commits together.
• Added tests.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
• All tests are passing.
• The new code doesn't generate RuboCop offenses.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.
• Updated cop documentation with rake generate_cops_documentation (required only when you've added a new cop or changed the configuration/documentation of an existing cop).

[bbatsov]

👍 Great work!