Filter out merge commits

**Is your feature request related to a problem? Please describe.**

I'd like to filter out merge commits like it can be done with `git log --no-merges`

**Describe the solution you'd like**

I don't know how this could be implemented best UX-wise. Maybe a new key binding that does exactly this ?
Another option would be to add support for arbitrary git log parameters. [tig](https://jonas.github.io/tig/) does this, which I like and used pretty much. You could start tig with `tig --no-merges`.

**Describe alternatives you've considered**

I don't see any alternative inside gitui, only using tig when I need to filter merge commits.

**Additional context**

Thanks for gitui, it's great !