Add missing test for 'git remote update --prune'

Signed-off-by: Björn Gustavsson <bgustavsson@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t5505-remote.sh

@@ -365,6 +365,17 @@ test_expect_success 'update with arguments' '
 
 '
 
+test_expect_success 'update --prune' '
+
+	(cd one &&
+	 git branch -m side2 side3) &&
+	(cd test &&
+	 git remote update --prune &&
+	 (cd ../one && git branch -m side3 side2)
+	 git rev-parse refs/remotes/origin/side3 &&
+	 test_must_fail git rev-parse refs/remotes/origin/side2)
+'
+
 cat > one/expect << EOF
   apis/master
   apis/side