Rewrite minimal theme using git-info
Rewrite minimal theme using git-info

As previous migrations of themes to git-info, this does not use its
verbose mode, so a repo with just untracked files is not considered
dirty.

It uses the newly-introduced `diverged` context and `coalesce` function
from git-info.

Closes #152

joshuarubin/dotfiles,Eriner/zim,joshuarubin/dotfiles,joshuarubin/dotfiles,joshuarubin/dotfiles