stack: add a 'stack()' revset We give access to the list of relevant commit with a revset. Benefits over 'topic(.)' are the filtering of obsolete changeset and the reordering (to topological as-in unstability were resolved).

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.