[PROF-10112] Also simplify actionview templates with three underscores

[ivoanjo]

What does this PR do?

This PR is a small fix on top of #3759. In that PR, we added code to detect method names dynamically generated by Rails actionview templates, slicing the parts of the method name that have random ids.

E.g. _app_views_layouts_explore_html_haml__2304485752546535910_211320 became _app_views_layouts_explore_html_haml.

When looking at one of our example applications, I realized that I was seeing methods that ended with both __number_number as well as ___number_number (three vs two underscores), e.g.:

• _app_views_articles_index_html_erb___2022809201779434309_12900
• _app_views_articles_index_html_erb__3816154855874711207_12920

On closer inspection of the actionview template naming code in https://github.com/rails/rails/blob/4fa56814f18fd3da49c83931fa773caa727d8096/actionview/lib/action_view/template.rb#L389:

    def method_name # :nodoc:
      @method_name ||= begin
        m = +"_#{identifier_method_name}__#{@identifier.hash}_#{__id__}"
        m.tr!("-", "_")
        m
      end
    end

... I realized that when @identifier.hash was a negative number, it would be printed including the - sign, which would be replaced by the tr into another _. (It's somewhat weird that they didn't just go with hash.abs but yeah... >_>).

Thus, there was a 50-50 chance that methods end up with three underscores, which would make them not merge together in the flamegraph.

This PR fixes this.

Motivation:

Make sure that these dynamically-generated methods merge together properly in a flamegraph.

Additional Notes:

Here's the issue in practice -- notice the purple frame ends with both _erb and _erb_:

...and here's how it looked without the previous patch as well:

How to test the change?

This change includes test coverage.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.91%. Comparing base (d9912ed) to head (52f6cbd).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3774   +/-   ##
=======================================
  Coverage   97.91%   97.91%           
=======================================
  Files        1243     1243           
  Lines       74829    74833    +4     
  Branches     3610     3610           
=======================================
+ Hits        73272    73276    +4     
  Misses       1557     1557           

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