cherry-pick: add test for `--skip` advice in `git commit`

In dcb500dc16 (cherry-pick/revert: advise using --skip, 2019-07-02),
`git commit` learned to suggest to run `git cherry-pick --skip` when
trying to cherry-pick an empty patch, but that was never tested for.

Here is a test that verifies that a message is given to the user that
contains the correct invocation.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t3510-cherry-pick-sequence.sh

@@ -123,7 +123,8 @@ test_expect_success 'revert --skip to skip commit' '
 test_expect_success 'skip "empty" commit' '
 	pristine_detach picked &&
 	test_commit dummy foo d &&
-	test_must_fail git cherry-pick anotherpick &&
+	test_must_fail git cherry-pick anotherpick 2>err &&
+	test_i18ngrep "git cherry-pick --skip" err &&
 	git cherry-pick --skip &&
 	test_cmp_rev dummy HEAD
 '