t6300: don't run cat-file on non-existent object
In t6300, some tests are guarded behind some prerequisites. Thus, objects created by those tests ain't available if those prerequisites are unsatistified. Attempting to run "cat-file" on those objects will run into failure. In fact, running t6300 in an environment without gpg(1), we'll see those warnings: fatal: Not a valid object name refs/tags/signed-empty fatal: Not a valid object name refs/tags/signed-short fatal: Not a valid object name refs/tags/signed-long Let's put those commands into the real tests, in order to: * skip their execution if prerequisites aren't satistified. * check their exit status code The expected value for objects with type: commit needs to be computed outside the test because we can't rely on "$3" there. Furthermore, to prevent the accidental usage of that computed expected value, BUG out on unknown object's type. Signed-off-by: Đoàn Trần Công Danh <congdanhqx@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>