[Fix #1177] Partially fix Rails/FilePath autocorrect for root filepat…

…hs by marking cop as unsafe for autocorrection

We correctly add a @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/>

config/default.yml

@@ -492,6 +492,7 @@ Rails/FilePath:
   VersionAdded: '0.47'
   VersionChanged: '2.4'
   EnforcedStyle: slashes
+  SafeAutoCorrect: false
   SupportedStyles:
     - slashes
     - arguments

lib/rubocop/cop/rails/file_path.rb

@@ -6,6 +6,9 @@ module Rails
       # Identifies usages of file path joining process to use `Rails.root.join` clause.
       # It is used to add uniformity when joining paths.
       #
+      # @safety
+      #   This cop's autocorrection is unsafe because it will replace the root path with an invalid path.
+      #
       # @example EnforcedStyle: slashes (default)
       #   # bad
       #   Rails.root.join('app', 'models', 'goober')