system_config: output most HOMEBREW_* variables.

Exclude the boring ones and exclude some that are already printed that
are just using their default values. HOMEBREW_PREFIX is the only one
where we’re always interested in the output to quickly communicate it.

Library/Homebrew/system_config.rb

@@ -169,10 +169,47 @@ class SystemConfig
       else
         f.puts "Core tap: N/A"
       end
+      defaults_hash = {
+        HOMEBREW_PREFIX: "/usr/local",
+        HOMEBREW_REPOSITORY: "/usr/local/Homebrew",
+        HOMEBREW_CELLAR: "/usr/local/Cellar",
+        HOMEBREW_CACHE: "#{ENV["HOME"]}/Library/Caches/Homebrew",
+      }.freeze
+      boring_keys = %w[
+        HOMEBREW_BROWSER
+        HOMEBREW_EDITOR
+
+        HOMEBREW_ANALYTICS_ID
+        HOMEBREW_ANALYTICS_USER_UUID
+        HOMEBREW_BREW_FILE
+        HOMEBREW_COMMAND_DEPTH
+        HOMEBREW_CURL
+        HOMEBREW_LIBRARY
+        HOMEBREW_MACOS_VERSION
+        HOMEBREW_RUBY_PATH
+        HOMEBREW_SYSTEM
+        HOMEBREW_OS_VERSION
+        HOMEBREW_PATH
+        HOMEBREW_PROCESSOR
+        HOMEBREW_PRODUCT
+        HOMEBREW_USER_AGENT
+        HOMEBREW_USER_AGENT_CURL
+        HOMEBREW_VERSION
+      ].freeze
       f.puts "HOMEBREW_PREFIX: #{HOMEBREW_PREFIX}"
+      if defaults_hash[:HOMEBREW_REPOSITORY] != HOMEBREW_REPOSITORY.to_s
         f.puts "HOMEBREW_REPOSITORY: #{HOMEBREW_REPOSITORY}"
+      end
+      if defaults_hash[:HOMEBREW_CELLAR] != HOMEBREW_CELLAR.to_s
         f.puts "HOMEBREW_CELLAR: #{HOMEBREW_CELLAR}"
-      f.puts "HOMEBREW_BOTTLE_DOMAIN: #{BottleSpecification::DEFAULT_DOMAIN}"
+      end
+      ENV.sort.each do |key, value|
+        next unless key.start_with?("HOMEBREW_")
+        next if boring_keys.include?(key)
+        next if defaults_hash[key.to_sym] == value
+        value = "set" if key =~ /(cookie|key|token)/i
+        f.puts "#{key}: #{value}"
+      end
       f.puts hardware if hardware
       f.puts "Homebrew Ruby: #{describe_homebrew_ruby}"
       f.puts "GCC-4.0: build #{gcc_4_0}" unless gcc_4_0.null?