More customizable smartlogs

[arxanas]

@claytonrcarter implemented this for v0.6.0-rc.1. Previously, git smartlog accepted a revset of which commits to show, but it was fairly limited in use due to #492. Now, you can provide useful revsets to get better views of the commit graph. Here's stack() | branches() | heads(draft()), which I might start using by default:

@claytonrcarter also mentions that it's useful to just show e.g. branches in the current stack with stack() & branches(), so that you understand the relative order of branches: