Fix a false positive of Style/FormatStringToken with unrelated format method call

[pocke]

Ref: #5483

Problem

Currently Style/FormatStringToken cop adds a falsy offense.

Time.now.strftime('%Y-%m-%d-%H-%M-%S')
format

$ rubocop
Inspecting 1 file
C

Offenses:

test.rb:1:26: C: Style/FormatStringToken: Prefer annotated tokens (like %<foo>s) over unannotated tokens (like %s).
Time.now.strftime('%Y-%m-%d-%H-%M-%S')
                         ^^

1 file inspected, 1 offense detected

But the cop does not add any offenses to the following code.

Time.now.strftime('%Y-%m-%d-%H-%M-%S')

$ rubocop
Inspecting 1 file
.

1 file inspected, no offenses detected

Cause

The cause is here.
https://github.com/bbatsov/rubocop/blob/86476f13e8e12e370f84911d82bef8638f3fdf96/lib/rubocop/cop/style/format_string_token.rb#L70-L77

The node.ancestors.last is a root node of ast. In other words, it equals processed_source.ast. So the code checks format method existence from whole a file.

Solution

Check format method existence only from ancestors of the string node.

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The more detailed you are, the better.

---

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[bbatsov]

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