List prepared branches

[b4mboo]

Since all the branches that have been prepared with git-review use the same kind of prefix, it should be quite simple to show a list active feature branches. We're looking for branches with the git-review prefix that have unmerged commits on them, but aren't associated with a (open or closed) pull request.

I imagine the output to look a bit like what we get from git review list. Columns should be date, feature name, branch name. Once issue #47 has been implemented, we could even add a column for the user name to that list. Sort either by date (default) or user name (again after #47).

This might be either a new command (i.e. git review branches) or a switch for an existing one (i.e. git review list --prepared).

Example use case:
Alice creates a feature branch and wants Bob to collaborate. She pushes the branch to remote, such that Bob can check it out. Since the repository they are working on has a lot of branches it is sometimes hard to find the correct branch right away. With this feature Bob could list all relevant branches to choose from.