Add "merged" state to merge indicators

[justinrobots]

When a local branch is associated with a PR that has already merged, the yellow merge target warning is displayed, which suggests that you rebase or merge. However, you don’t actually want to rebase or merge in this situation because the work is already done.

We should change the indicator in this state to show that the remote has been merged.

From @eamodio :

I wanted to get "merged" state into that status, but couldn't make it. And if the branch looks merged, we would offer the clean up actions (and need to change the indicator accordingly)