coloured git-prompt: paint detached HEAD marker in red

Paint the marker for normal state in green and detached state
in red, instead of the other way around.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-prompt.sh

@@ -335,7 +335,7 @@ __git_ps1 ()
 				local flags_color="$c_lblue"
 				local branchstring="$c${b##refs/heads/}"
 
-				if [ $detached = yes ]; then
+				if [ $detached = no ]; then
 					branch_color="$ok_color"
 				else
 					branch_color="$bad_color"