Git don't throw up when not on some branch

[spruce]

Hi, thanks for the best dotfiles yet 👍 . Now to my "problem". Currently when being in a detached HEAD state the prompt throws up and always shows a warning that

fatal: ref HEAD is not a symbolic ref

and shows an incomplete prompt e.g.:

🔌  ⚡️in .dotfiles/ on

This PR fixes that and displays:

🔌  ⚡️  in .dotfiles/ on detached HEAD

Maybe the message could be tweak a little (maybe show commit sha) but is in my opinion way better than throwing an error.