This commit removes the banner displayed on Eshell launch. I will
maybe do something with it one day similar to my fish_greeting
function for the fish shell.
The git prompt now also separates the current path in half if we are
in a git repository: the path up to the repository, the git prompt,
and then the relative path to the repo is are shown in that order.
To achieve that, `phundrak/is-dir-a-git-repo' has been modified to
return either the path to the git repo or `nil'.