Don't require "branch: main" for resources with revisions set

Just like when a tag is set, when a revision is set for a resource it shouldn't _also_ specify that it comes from the main branch.

Library/Homebrew/resource_auditor.rb

@@ -171,6 +171,7 @@ module Homebrew
       return if spec_name != :head
       return unless Utils::Git.remote_exists?(url)
       return if specs[:tag].present?
+      return if specs[:revision].present?
 
       branch = Utils.popen_read("git", "ls-remote", "--symref", url, "HEAD")
                     .match(%r{ref: refs/heads/(.*?)\s+HEAD})&.to_a&.second