Add allowlist, make branch: mandatory

Library/Homebrew/formula_auditor.rb

@@ -523,9 +523,15 @@ def audit_specs
         spec_name = name.downcase.to_sym
         next unless (spec = formula.send(spec_name))
 
+        except = @except.to_a
+        if spec_name == :head &&
+           tap_audit_exception(:head_non_default_branch_allowlist, formula.name, spec.specs[:branch])
+          except << "head_branch"
+        end
+
         ra = ResourceAuditor.new(
           spec, spec_name,
-          online: @online, strict: @strict, only: @only, except: @except
+          online: @online, strict: @strict, only: @only, except: except
         ).audit
         ra.problems.each do |message|
           problem "#{name}: #{message}"

Library/Homebrew/resource_auditor.rb

@@ -125,18 +125,17 @@ def audit_urls
     end
 
     def audit_head_branch
-      return if !@online || !@strict || spec_name != :head || !Utils::Git.remote_exists?(url)
+      return unless @online
+      return unless @strict
+      return unless spec_name == :head
+      return unless Utils::Git.remote_exists?(url)
 
       branch = Utils.popen_read("git", "ls-remote", "--symref", url, "HEAD")
                     .match(%r{ref: refs/heads/(.*?)\s+HEAD})[1]
 
-      return if branch == "master" && specs[:branch].blank? || branch == specs[:branch]
+      return if branch == specs[:branch]
 
-      if branch == "master"
-        problem "Remove `branch: \"#{specs[:branch]}\"`"
-      else
-        problem "Use `branch: \"#{branch}\"` to specify the correct default branch"
-      end
+      problem "Use `branch: \"#{branch}\"` to specify the default branch"
     end
 
     def problem(text)