show: implement "stack" view People often want to know what they are working on *now*. As part of this, they also commonly want to know how that work is related to other changesets in the repo so they can perform common actions like rebase, histedit, and merge. `hg show work` made headway into this space. However, it is geared towards a complete repo view as opposed to just the current line of work. If you have a lot of in-flight work or the repo has many heads, the output can be overwhelming. The closest thing Mercurial has to "show me the current thing I'm working on" that doesn't require custom revsets is `hg qseries`. And this requires MQ, which completely changes workflows and repository behavior and has horrible performance on large repos. But as sub-optimal as MQ is, it does some things right, such as expose a model of the repo that is easy for people to reason about. This simplicity is why I think a lot of people prefer to use MQ, despite its shortcomings. One common development workflow is to author a series of linear changesets, using bookmarks, branches, anonymous heads, or even topics (3rd party extension). I'll call this a "stack." You periodically rewrite history in place (using `hg histedit`) and reparent the stack against newer changesets (using `hg rebase`). This workflow can be difficult because there is no obvious way to quickly see the current "stack" nor its relation to other changesets. Figuring out arguments to `hg rebase` can be difficult and may require highlighting and pasting multiple changeset nodes to construct a command. The goal of this commit is to make stack based workflows simpler by exposing a view of the current stack and its relationship to other releant changesets, notably the parent of the base changeset in the stack and newer heads that the stack could be rebased or merged into. Introduced is the `hg show stack` view. Essentially, it finds all mutable changesets from the working directory revision in both directions, stopping at a merge or branch point. This limits the revisions to a DAG linear range. The stack is rendered as a concise list of changesets. Alongside the stack is a visualization of the DAG, similar to `hg log -G`. Newer public heads from the branch point of the stack are rendered above the stack. The presence of these heads helps people understand the DAG model and the relationship between the stack and changes made since the branch point of that stack. If the "rebase" command is available, a `hg rebase` command is printed for each head so a user can perform a simple copy and paste to perform a rebase. This view is alpha quality. There are tons of TODOs documented inline. But I think it is good enough for a first iteration.