git-prompt.sh: do not print duplicate clean color code

Do not print a duplicate clean color code when there
is no other indicators other than the current branch
in colored prompt.

Acked-by: SZEDER Gábor <szeder@ira.uka.de>
Signed-off-by: Eduardo R. D'Avila <erdavila@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-prompt.sh

@@ -253,8 +253,8 @@ __git_ps1_colorize_gitstring ()
 		branch_color="$bad_color"
 	fi
 	c="$branch_color$c"
-	b="$b$c_clear"
 
+	z="$c_clear$z"
 	if [ "$w" = "*" ]; then
 		w="$bad_color$w"
 	fi