Panic when displaying log of an empty branch

Trying to display the log on an empty branch results in a panic.

Steps to reproduce:

- mkdir example
- cd example
- git init
- gitu (and press `l l` to show the log of the current branch)

Error message:

```
panicked at /home/foo/.cargo/registry/src/index.crates.io-6f17d22bba15001f/gitu-0.25.0/src/screen/mod.rs:82:36:
index out of bounds: the len is 0 but the index is 0
```