Linter: Improve unextractable tags in erb-no-duplicate-branch-elements

[marcoroth]

This pull request updates erb-no-duplicate-branch-elements to stop reporting shared tags that cannot actually be extracted, and makes the linter CLI stop advertising those offenses as autocorrectable.

<% if foo %>
  <p class="mt-3 text-balance">
    Content
  </p>

  <%= link_to "Link", other_path %>
<% else %>
  <p class="mt-3 text-balance">
    Other content
  </p>

  <%= link_to "Other link", other_path %>
<% end %>

which produced two hints plus Fixable 2 offenses | 2 autocorrectable using --fix, while --fix left the file untouched.

When the content of a shared tag differs between branches, the only way to extract it is to wrap the whole conditional in that tag.

The rule reported the hint without checking that this rewrite was available. In the template above the <p> has a sibling link_to that has to stay inside the conditional, so wrapping would swallow the links too. The only remaining "fix" is to split the conditional into several, which duplicates the condition instead of the markup.

Resolves #1570

[natematykiewicz]

Does this also resolve #1878?

[marcoroth]

@natematykiewicz looks like it does. On current main they are not reported anymore: Playground