color: add the ability to display configured style to 'debugcolor' The 'hg debugcolor' command gains a '--style' flag to display all the configured labels and their styles. This have many benefits: * discovering documented label, * checking consistency between label's style, * showing the actual style of a label.

color: sort output of 'debugcolor' The previous ordering were provided by the set. The new output is more stable and rational. In addition we have some logic to keep the '_background' version together to help readability.