Check for duplicate content in relevant section when inserting into files

[excid3]

I ran into some unexpected behavior today with insert_into_file. I was trying to insert , null: false into a migration after some text, but nothing happened.

  t.belongs_to :recipient, null: false
  t.string :type

insert_into_file path, ", null: false", after: "t.string :type"

This does nothing because , null: false exists in the file (anywhere).

Using after: I would expect insert_into_file to only care if the string exists after the matching line, not before it.

This PR would change the behavior to only look through the before or after portions of the file content to make it more intuitive.

[rafaelfranca]

I'll have to revert this PR since it changes the behavior when the regexp contains capture. split returns the capture inside the array of split strings, so we can't know with the current implementation what is the after and before the regexp.