At the beginning of __git_ps1, right after determining that the
function is running in pc mode, set PS1 to a plain (undecorated)
prompt. This makes it possible to simply return early without having
to set PS1 if the prompt should not be decorated.
Signed-off-by: Richard Hansen <rhansen@bbn.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>