A Rails view may rely on several templates (e.g. layouts and partials)
in addition to the template for the action being rendered (e.g.
"show.html.erb"). To track which view file is currently being rendered
for the purpose of generating template tree digests used in cache
fragment keys, Action View uses a stack, the top item of which is
accessed via the @current_template variable (introduced in 1581cab9).

Consider the following template:

    <!-- home.html.erb -->
    <%= render layout: "wrapper" do %>
      <%= cache "foo" %>
        HOME
      <%= end %>
    <%= end %>

Inside the block passed to the render helper, @current_template
corresponds to the wrapper.html.erb template instead of home.html.erb.
As wrapper.html.erb is then used as the root node for generating the
template tree digest used in the cache fragment key, the cache fragment
fails to expire upon changes to home.html.erb. Additionally, should a
second template use the wrapper.html.erb layout and contain a cache
fragment with the same key, the cache fragment keys for both templates
will be identical - causing cached content to "leak" from one view to
another (as described in #38984).

This commit skips adding templates to the stack when rendered as a
layout with a block via the render helper, ensuring correct and unique
cache fragment digests. Additionally, the virtual_path keyword arguments
found in CacheHelper and all references to the are removed as they no
longer possess any function. (Following the introduction of
@current_template, virtual_path is accessed via
@current_template.virtual_path rather than as a standalone variable.)