Show which branches have unmerged changes

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

I use feature branches. Sometimes I work on one feature before switching to another. With lots of branches, I sometimes lose track of which branches still need work done, and which branches have already been merged and can be safely deleted.

**Describe the solution you'd like**

It would be nice if there was some indicator in the "Local Branches" pane for the branches that have not yet been merged into the branch that's currently checked out.

**Additional context**

I tried sketching out a patch [(WIP)](https://github.com/c2d7fa/lazygit), and came up with this:


The deltas next to some branches indicate that the branch has not yet been merged into the checked out branch. Branches that don't have deltas have been merged and can be safely deleted.

I'd be happy to try and turn this into a proper pull request if this seems like a good fit for the project.

By the way, thanks for the great work! Lazygit is a really cool project!