It's useful to know when you've modified a branch that exists on a
remote. A typical case is when you have pushed a branch to a remote
and then rewritten it. This commit adds an indication in the
`branches` template keyword. A branch that needs to be pushed to a
remote now has a `*` at the end (similar to how conflicted branches
have a `?` at the end). Note that the indication only considers
remotes where the branch currently exists, so there won't be an
indication that the branch has not been pushed to a remote.
Closes#254
