Show merge status of local branches

[ghost]

It would be great if lab could show the merge status of local branches, similar to how Phabricator Arcanist's arc branch (without arguments) works. See e.g. the example about "the handy arc branch" (search on page) in Will Demaine's article on how to use Phabricator's code review feature.

The output of arc branch is in turn similar to how git branch -vv shows the differences between the local and remote branches, in case you have no chance to experiment with arc branch yourself.