test: mark a regression in Git

The issue is reported, and a proposed fix has been sent:

  https://marc.info/?l=git&m=155961441427321

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>

test/main.t

@@ -853,7 +853,7 @@ test_expect_failure 'push updates notes' '
 	test_cmp expected actual
 '
 
-test_expect_success 'pull tags' '
+test_expect_unstable 'pull tags' '
 	test_when_finished "rm -rf hgrepo gitrepo" &&
 
 	(