config: include core branch in output

Library/Homebrew/system_config.rb

@@ -44,6 +44,10 @@ class SystemConfig
       CoreTap.instance.git_last_commit || "never"
     end
 
+    def core_tap_branch
+      CoreTap.instance.git_branch || "(none)"
+    end
+
     def core_tap_origin
       CoreTap.instance.remote || "(none)"
     end
@@ -116,6 +120,7 @@ class SystemConfig
         f.puts "Core tap ORIGIN: #{core_tap_origin}"
         f.puts "Core tap HEAD: #{core_tap_head}"
         f.puts "Core tap last commit: #{core_tap_last_commit}"
+        f.puts "Core tap branch: #{core_tap_branch}"
       else
         f.puts "Core tap: N/A"
       end