prompt: fix untracked files for zsh

prompt: fix untracked files for zsh

We signal presense of untracked files by adding a per-cent sign '%'
to the prompt.  But because '%' is used as an escape character to
introduce prompt customization in zsh (just like bash prompt uses
'\' to escape '\u', '\h', etc.), we need to say '%%' to get a
literal per-cent.

Helped-by: Andreas Schwab <schwab@linux-m68k.org>
Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>