completion: prompt: fix color for Zsh

We don't need PROMPT_COMMAND in Zsh; we are already using %F{color} %f,
which in turn use %{ and %}, which are the equivalent of Bash's
\[ and \].

We can use as many colors as we want and output directly into PS1
(or RPS1) without the risk of buffer wrapping issues.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-prompt.sh

@@ -97,7 +97,8 @@
 # If you would like a colored hint about the current dirty state, set
 # GIT_PS1_SHOWCOLORHINTS to a nonempty value. The colors are based on
 # the colored output of "git status -sb" and are available only when
-# using __git_ps1 for PROMPT_COMMAND or precmd.
+# using __git_ps1 for PROMPT_COMMAND or precmd in Bash,
+# but always available in Zsh.
 #
 # If you would like __git_ps1 to do nothing in the case when the current
 # directory is set up to be ignored by git, then set
@@ -553,9 +554,11 @@ __git_ps1 ()
 
 	local z="${GIT_PS1_STATESEPARATOR-" "}"
 
-	# NO color option unless in PROMPT_COMMAND mode
-	if [ $pcmode = yes ] && [ -n "${GIT_PS1_SHOWCOLORHINTS-}" ]; then
-		__git_ps1_colorize_gitstring
+	# NO color option unless in PROMPT_COMMAND mode or it's Zsh
+	if [ -n "${GIT_PS1_SHOWCOLORHINTS-}" ]; then
+		if [ $pcmode = yes ] || [ -n "${ZSH_VERSION-}" ]; then
+			__git_ps1_colorize_gitstring
+		fi
 	fi
 
 	b=${b##refs/heads/}