Consider showing the branch name in the files panel? 

Currently it's impossible to figure the branch name when:

1. in a `nil` file like it was deleted, right side of a diff
2. in the left side when it was newly created as it's `nil`
3. or in the files pane `DiffviewFiles`

because they are not a file on disk, so we can't to find a git root. 

Plugins like `lua-line` also can't find the git root, so the branch name disapear. 