Stashed support for fish

gitprompt.fish wasn't updated to support the stashed status, and also didn't show the clean status (as the line 7 returned by gitstatus.py is not the clean status, but now it is stashed).

I have added the staged status to mimic the behaviour in bash, also changed __GIT_PROMPT_DIR from .bash to .gitprompt so the script can work out of the box.

The changes where tested on fish 2.1.0 on a Debian like system (Lihuen).