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.

color: extract color and effect display from 'debugcolor' We are about to introduce a second mode for 'hg debugcolor' that would list the known label and their configuration, so we split the code related to color and effect out of the main function.

color: restore _style global after debugcolor ran Before this change, running 'debugcolor' would destroy all color style for the rest of the process life. We now properly backup and restore the variable content. Using a global variable is sketchy in general and could probably be removed. However, this is a quest for another adventure.