t5004: resurrect original empty tar archive test

Add a test to verify the emptiness of an archive by extracting its
contents.  Don't run this test if the version of tar doesn't support
archives containing only a comment header, though.

The existing check 'tar archive of empty tree is empty' used to work
like that (minus the tar capability check) but was changed to depend
on the exact representation of empty tar files created by git archive
instead of on the behaviour of tar in order to avoid issues with
different tar versions.

The different approaches test different things: The existing one is
for empty trees, for which we know the exact expected output and thus
we can simply check it without extracting; the new one is for commits
with empty trees, whose archives include stamps and so the more
"natural" check by extraction is a better fit because it focuses on
the interesting aspect, namely the absence of any archive entries.
Signed-off-by: NRene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

t/t5004-archive-corner-cases.sh

@@ -27,6 +27,20 @@ check_dir() {
 	test_cmp expect actual
 }
 
+# bsdtar/libarchive versions before 3.1.3 consider a tar file with a
+# global pax header that is not followed by a file record as corrupt.
+if "$TAR" tf "$TEST_DIRECTORY"/t5004/empty-with-pax-header.tar >/dev/null 2>&1
+then
+	test_set_prereq HEADER_ONLY_TAR_OK
+fi
+
+test_expect_success HEADER_ONLY_TAR_OK 'tar archive of commit with empty tree' '
+	git archive --format=tar HEAD >empty-with-pax-header.tar &&
+	make_dir extract &&
+	"$TAR" xf empty-with-pax-header.tar -C extract &&
+	check_dir extract
+'
+
 test_expect_success 'tar archive of empty tree is empty' '
 	git archive --format=tar HEAD: >empty.tar &&
 	perl -e "print \"\\0\" x 10240" >10knuls.tar &&