resource_auditor: add audit for HEAD default branch

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

@@ -124,6 +124,20 @@ def audit_urls
       end
     end
 
+    def audit_head_branch
+      return unless @online
+      return unless @strict
+      return if 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 == specs[:branch]
+
+      problem "Use `branch: \"#{branch}\"` to specify the default branch"
+    end
+
     def problem(text)
       @problems << text
     end