[Emacs] Eshell prompt update

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'.

.spacemacs

@@ -149,7 +149,8 @@ This function is called at the very end of Spacemacs initialization."
     (cons 320 "#6f4e52")
     (cons 340 "#5B6268")
     (cons 360 "#5B6268")))
- '(vc-annotate-very-old-color nil))
+ '(vc-annotate-very-old-color nil)
+ '(warning-suppress-log-types '((comp))))
 (custom-set-faces
  ;; custom-set-faces was added by Custom.
  ;; If you edit it by hand, you could mess it up, so be careful.