Show pushed/merged statuses of branches

**Is your feature request related to a problem? Please describe.**
Sometimes I want to go and delete merged branches (that is: branches merged to my main branch). But I can't know that a given branch is merged without hitting enter on it to view its commits.

If I expand the branches window I can see commit hashes against the branches but they're all just coloured white.

**Describe the solution you'd like**
Colour the commit hashes in the branches view in the same way we colour those of commit views.

This is good because it'll be consistent but you'll need to enlarge the window to be able to see them.

I considered having a separate tick icon against the branch if it's merged but that sounds too noisy.

I've got a WIP branch for this: https://github.com/jesseduffield/lazygit/tree/show-merged-status