[Fix #1071] Mark Rails/FilePath unsafe because of interaction with root filepath

[corsonknowles]

Here, we add the requested @safety annotation for Rails/FilePath since it can produce invalid code like this:

pass any string argument beginning with '/' to Rails.root.join and it will omit the Rails path

(dev)> Rails.root.join '/'
=> #<Pathname:/>
(dev)> Rails.root.join '//'
=> #<Pathname://>
(dev)> Rails.root.join '/abc'
=> #<Pathname:/abc>
(dev)> Rails.root.join '/abc/'
=> #<Pathname:/abc/>

This partly addresses: #1071

See:

• FilePath safe autocorrect breaks code #1071
  And also:
• Rails/FilePath autocorrect shouldn't remove important trailing slash in dynamic string containing Rails.root #1177

See also:

• Rails.root.join '/' returns #<Pathname:/> rails/rails#53578

This will not be amended in Rails, so the responsibility for the autocorrect behavior falls on Rubocop.

---

Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.
• If this is a new cop, consider making a corresponding update to the Rails Style Guide.

[corsonknowles]

This came up while I was working on fixing all safely auto-correctable Rubocop rules in our code base.

Example problems:

relative_path = absolute_path.delete_prefix("#{Rails.root}/") # rubocop:disable Rails/FilePath

path&.start_with?("#{Rails.root}/") # rubocop:disable Rails/FilePath

[corsonknowles]

@Earlopain Are we doing the right thing here?

[koic]

Can you add concrete example code in the safety section of the documentation and add a changelog entry?

[koic]

Suggested change

	VersionChanged: '2.4'
	VersionChanged: '<<next>>'

[koic]

This PR will be closed to take the approach of ignoring unsafe cases proposed in #1433. Thank you.