Always suggest a HEAD branch name if we can find one

- If a HEAD branch name isn't specified at all, then the user probably
  wants to shortcut adding one by being told what the default branch for
  the repo is. Otherwise they have to click the URL, look at the GitHub
  UI, then type the branch name into `branch: "foo"` syntax.

Library/Homebrew/resource_auditor.rb

@@ -178,23 +178,22 @@ module Homebrew
     end
 
     def audit_head_branch
+      return unless @online
       return if spec_name != :head
       return if specs[:tag].present?
       return if specs[:revision].present?
       # Skip `resource` URLs as they use SHAs instead of branch specifiers.
       return if name != owner.name
       return unless url.end_with?(".git")
-
-      problem "Git `head` URL must specify a branch name" if specs[:branch].blank?
-
-      return unless @online
       return unless Utils::Git.remote_exists?(url)
 
       detected_branch = Utils.popen_read("git", "ls-remote", "--symref", url, "HEAD")
                              .match(%r{ref: refs/heads/(.*?)\s+HEAD})&.to_a&.second
-      return if detected_branch.blank? || detected_branch == specs[:branch]
 
-      problem "Detected a default branch \"#{detected_branch}\", not \"#{specs[:branch]}\""
+      message = "Git `head` URL must specify a branch name"
+      message += " - try `branch: \"#{detected_branch}\"`" if detected_branch.present?
+
+      problem message if specs[:branch].blank? || detected_branch != specs[:branch]
     end
 
     def problem(text)

Library/Homebrew/test/formula_auditor_spec.rb

@@ -715,16 +715,17 @@ RSpec.describe Homebrew::FormulaAuditor do
     end
 
     it "requires `branch:` to be specified for Git head URLs" do
-      fa = formula_auditor "foo", <<~RUBY
+      fa = formula_auditor "foo", <<~RUBY, online: true
         class Foo < Formula
           url "https://brew.sh/foo-1.0.tgz"
           sha256 "31cccfc6630528db1c8e3a06f6decf2a370060b982841cfab2b8677400a5092e"
-          head "https://github.com/example/foo.git"
+          head "https://github.com/Homebrew/homebrew-test-bot.git"
         end
       RUBY
 
       fa.audit_specs
-      expect(fa.problems.first[:message]).to match("Git `head` URL must specify a branch name")
+      # This is `.last` because the first problem is the unreachable stable URL.
+      expect(fa.problems.last[:message]).to match("Git `head` URL must specify a branch name")
     end
 
     it "suggests a detected default branch for Git head URLs" do
@@ -732,17 +733,31 @@ RSpec.describe Homebrew::FormulaAuditor do
         class Foo < Formula
           url "https://brew.sh/foo-1.0.tgz"
           sha256 "31cccfc6630528db1c8e3a06f6decf2a370060b982841cfab2b8677400a5092e"
-          head "https://github.com/Homebrew/homebrew-core.git", branch: "master"
+          head "https://github.com/Homebrew/homebrew-test-bot.git", branch: "master"
+        end
+      RUBY
+
+      message = "Git `head` URL must specify a branch name - try `branch: \"main\"`"
+      fa.audit_specs
+      # This is `.last` because the first problem is the unreachable stable URL.
+      expect(fa.problems.last[:message]).to match(message)
+    end
+
+    it "ignores a pre-existing correct HEAD branch name" do
+      fa = formula_auditor "foo", <<~RUBY, online: true
+        class Foo < Formula
+          url "https://brew.sh/foo-1.0.tgz"
+          sha256 "31cccfc6630528db1c8e3a06f6decf2a370060b982841cfab2b8677400a5092e"
+          head "https://github.com/Homebrew/homebrew-test-bot.git", branch: "main"
         end
       RUBY
 
       fa.audit_specs
-      # This is `.last` because the first problem is the unreachable stable URL.
+      expect(fa.problems).not_to match("Git `head` URL must specify a branch name")
-      expect(fa.problems.last[:message]).to match('Detected a default branch "main", not "master"')
     end
 
     it "ignores `branch:` for non-Git head URLs" do
-      fa = formula_auditor "foo", <<~RUBY
+      fa = formula_auditor "foo", <<~RUBY, online: true
         class Foo < Formula
           url "https://brew.sh/foo-1.0.tgz"
           sha256 "31cccfc6630528db1c8e3a06f6decf2a370060b982841cfab2b8677400a5092e"
@@ -755,7 +770,7 @@ RSpec.describe Homebrew::FormulaAuditor do
     end
 
     it "ignores `branch:` for `resource` URLs" do
-      fa = formula_auditor "foo", <<~RUBY
+      fa = formula_auditor "foo", <<~RUBY, online: true
         class Foo < Formula
           url "https://brew.sh/foo-1.0.tgz"
           sha256 "31cccfc6630528db1c8e3a06f6decf2a370060b982841cfab2b8677400a5092e"
@@ -768,7 +783,7 @@ RSpec.describe Homebrew::FormulaAuditor do
       RUBY
 
       fa.audit_specs
-      expect(fa.problems).to be_empty
+      expect(fa.problems).not_to match("Git `head` URL must specify a branch name")
     end
 
     it "allows versions with no throttle rate" do