topic: add color for the active topic We reuse the same color that the one used for the active bookmark.

topic: add formatter support Using formatter to output the list of topic will add support for color and richer templater. We add some test for output control. We introduce a dedicated test to help with coming more complexe output.

topic: extract the code listing all topics The code will become more complexe with formatter support and additional data in the verbose output. We extract the code in its own function first.

stack: introduce and explicite command to display the stack If is worth testing having the feature lives outside 'hg topic --list'. The 'stack' name is sub optimal because the conflict with 'status'. using "hg st" would still work as 'st' is hard coded, but this mean 'hg stack' has no short version.

testedwith: update to 3.9 This is might still 3.7 compatible but I'm no longer running the test and I would says the user base is small enough to not care.

topic: make sure we have the 'wlock' when setting topic We where writing the '.hg/topic' file without a lot. That was bad.