feature request: base branches in gtm list output

It would be nice to have some sort of info about head and base branches for tasks which already correspond to a pull request (which are the vast majority, at least in my case). Some sort of tree output would be just great, similar to what `tree` gives you for folders:

```
.
├── oh
│   ├── argh
│   └── eh
└── uuh
    ├── maybe
    └── noo
        └── yes

```