Show unpushed commits in other branches which aren't checked out

[aleung]

Clustergit rightnow only reports unpushed commits in the current branch of a repo. Could it add support to find all unpushed commits in other branches?

My use case: I often work on multiple branches at the same time, for example bug fixes and new features, and it's easy to leave some commits in a branch forgot to push to upstream. I can use command git --no-pager log --branches --not --remotes --no-walk --decorate --oneline to list branches containing unpushed commits in a repo. It's good to make it works on multiple repos by clustergit.