Mercurial branches are not "cheap". Unlike Git, to create or destroy a branch, you actually have to commit a changeset to the repository. Branches are great for…something…but whatever that is, we're not doing it.
Enter Bookmarks.
Bookmarks work similarly to the Git notion of branches. To create a bookmark, you do not create a changeset. A bookmark is simply a reference to a changeset. It ires repository meta-data, and by default stays "local" (in your working copy).