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: NAndreas Schwab <schwab@linux-m68k.org>
Signed-off-by: NFelipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

contrib/completion/git-prompt.sh

@@ -338,7 +338,7 @@ __git_ps1 ()
 			   [ "$(git config --bool bash.showUntrackedFiles)" != "false" ] &&
 			   [ -n "$(git ls-files --others --exclude-standard)" ]
 			then
-				u="%"
+				u="%${ZSH_VERSION+%}"
 			fi
 
 			if [ -n "${GIT_PS1_SHOWUPSTREAM-}" ]; then