stack: display the base of the stack Displaying the first parent of the stack seems useful. I'm even tempted to call it 't0' and have the topic still active when 'hg up t0' is used to move to the base. As a side effect we gain a way to denote that the stack is not linear. I'm not super convinced that it is the right way to display it, but this is better than no information.

stack: add some default color configuration We can make the output more insightful with some default colors, let's do it.

stack: add basic formatter and label support This still is not great, especially I would like '-T' to be able to control how we display the changeset, but this is useful progress.

stack: show the currently active changeset and unstable ones Still super basic, but give a basic idea of the feature. We use both symbols and explicit text because symbols are cool but text is more explicit

stack: add "t" prefix to index in the output This make the existance and meaning of "t2" reference clearer.

stack: allow to refer to changeset using "t2" form hg up "t0" is seen as "update to the first changeset of my current topic". Eventually we'll drop the "t2" form in favor of the planned generic indexing operator '.{t2}'.