t6300: don't run cat-file on non-existent object

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>